Tarsnap performance issues in late March, most of April

http://mail.tarsnap.com/tarsnap-announce/msg00031.html

by pndmnm • 11 years ago

172 113 11 years ago

mail.tarsnap.com

mtsmith853• 11 years ago

This line: I would have sent out an email to the mailing lists earlier; but since at each point I thought I was "one change away" from fixing the problems, I kept on delaying said email until it was clear that the problems were finally fixed" is such a common situation for most people, but I tend to see it with engineers especially. I find I struggle with it an incredible amount. In some ways, I guess it seems healthy or reassuring that incredibly smart people like Colin Percival suffer from similar challenges around fully understanding the scope of the problem and the solution.

All that being said, I really respect the detailed response from a technical perspective as well as owning up to (and the decisions that went into) a spell of downgraded performance.

Later edit because I don't want to spam the comments: I'd love some context (maybe from cperciva himself?) around the performance enhancement of integrating new Intel AESNI instructions. This is well beyond my depth and while Colin mentions that it didn't necessarily increase performance, I'm wondering if the hope is it would longterm? Or were there other benefits to such an integration?

Poiesis• 11 years ago

I came here to write this comment essentially.

Very much of the time I feel, "If I knew what the problem[s] [was|were] it'd be solved by now!" That's not exactly true of course but of course diagnosis is a large part of the total solution.

This type of an answer that Colin gave above does not exactly win friends and influence people in most situations where you're part of a team or hierarchy. Can anyone share what they've done to give better answers in these cases? I understand why people want the answers, but I don't have them to give right away particularly when it's Someone Else's system.

cperciva18• 11 years ago

Just to be clear, when Tarsnap users wrote to me I told them everything I could. The "I think it will be fixed soon" delay in sending out an email to the lists affected only people who didn't notice or noticed but didn't ask about the issue.

jballanc• 11 years ago

One trick that I've learned (though I still have trouble routinely applying it myself) for these situations is: less is more.

That is, as engineers we tend to want details. All the details. We want to know what happened, why it happened, how it's going to be fixed, and how long that will take. Because we want all that detail for ourselves, we hesitate to contact our customers/boss until we have all the details. Combine that with a desire to fix problems as they come up, and you end up with, "I never told you there was a problem because I was always one fix away from the solution."

But most people are not engineers. They want to be acknowledged. They want to feel informed, even if they have less details than what you would like to provide for them. Sometimes, something as simple as, "We've noticed that there is an issue and are currently working on a fix," goes a long way. Also don't be afraid to pull out, "Users have been reporting issues with backup performance. We do not currently believe this represents a service failure, but we are working to return performance to normal levels."

Your users trust you (otherwise they wouldn't pay you). If you "believe" something, they will too.

cperciva18• 11 years ago

I would have sent out an email to the mailing lists earlier; but since at each point I thought I was "one change away" from fixing the problems, I kept on delaying said email until it was clear that the problems were finally fixed

This ties in to the last lesson I mentioned at the bottom:

5. When performance drops, it's not always due to a single problem; sometimes there are multiple interacting bottlenecks.

Every time I identified a problem, I was correct that it was a problem -- my failing was in not realizing that there were several things going on at once.

mtsmith853• 11 years ago

That totally jibes with what I found "reassuring" in a sense. That even very smart people sometimes get hit with inadvertent "multiple problems looking like a single issue" situations.

jcrites• 11 years ago

Every time I identified a problem, I was correct that it was a problem -- my failing was in not realizing that there were several things going on at once.

Very common! One thing that's been helpful for us is establishing predefined system performance thresholds that, if exceeded, initiate the chain of events that will lead to customer communication. "If X% of requests are failing, then we had better advertise that the system is degraded." Discussing and setting these thresholds in advance and the expectation that they'll result in communication helps drive the right outcome. It's not perfect, because one is always tempted to make a judgment call in the circumstance, which is vulnerable to the same effect, but it's a good start.

Thanks for sharing!

spydum• 11 years ago

i tend to get to debug problems like this (usually in 3rd party code i dont know the internals of) pretty frequently.. my experience has been it tends to follow a curve..MOST of the time, the problem is simple and you can quickly dispatch it. the scary (or fun, depending on your perspective) part hits when you pass the first level, and there are still problems.. and you dont know if it's two or ten levels deeper. then you get into that crazy test/optimize cycle and crawl out two weeks later wondering when you last ate..

cperciva18• 11 years ago

I'd love some context (maybe from cperciva himself?) around the performance enhancement of integrating new Intel AESNI instructions.

I was using OpenSSL for that (which was using a software implementation). The code (you can see it in spiped) now detects the CPU feature and selects between AESNI or OpenSSL automatically. Given that the tarsnap server code was spending about 40% of its time running AES, it's a nontrivial CPU time saving.

I should probably have been clearer in my writeup though -- using AESNI was never a "once I roll this out everything will be good" fix. Rather, it was a case of "I have this well-tested code available which will help a bit while I finish testing the real fixes".

gonzo• 11 years ago

One wonders why you aren't using a version of OpenSSL that has the AESNI bits already in it.

mryan• 11 years ago

This "it's almost fixed, I'll email the client soon" pattern is something I have personally struggled with a lot, and I agree it appears to be common with engineers.

My workaround has been to make something else responsible for sending the email. In a team, this could be a manager setting a cut-off point after which communication must be made. When working on my own, I set an alarm for X minutes. When that alarm goes off I ignore the internal voice which says "just try one more thing, then send the email", and send an update to let the relevant people know my current progress, ETA to fix, and when they can expect the next update.

I think this is similar to how GTD encourages us to use systems for storing to-do lists instead of trying to remember them - our fragile human brains are not always to be trusted.

patio113• 11 years ago

In case any other customer is wondering "Wait, I didn't hear anything from my monitoring about that and I'm retroactively worried. How worried should I be?" like I was: I just pulled our logs and reconstructed them, and it shows over the last ~30 days that the worse-case performance of our daily backup (~150 MB per day delta, ~45 GB total post deduplication) was about 40% longer than our typical case. This didn't trip our monitoring at the time because they all completed successfully.

n.b. Our backups run outside of the hotspot times for Tarsnap, so we may have had less performance impact than many customers. I have an old habit of "Schedule all cron jobs to start predictably but at a random offset from the hour to avoid stampeding any previously undiscovered SPOFs." That's one of the Old Wizened Graybeard habits that I picked up from one of the senior engineers at my last real job, which I impart onto y'all for the same reason he imparted it onto me: it costs you nothing and will save you grief some day far in the future.

mtsmith853• 11 years ago

Hear hear on said Old Wizened Graybeard habit. The amount of pain inflicted from twenty jobs all starting up at :00 (or even :30, :45, etc.) when they could easily run at :04 or :17 can be huge. Anecdotally I once "lost" a sandbox server to a ton of developer sandbox jobs starting at :00 and not completing before the next batch started.

protomyth2• 11 years ago

Funny part to that, was on a project with multiple teams with multiple crontabs. Each team took that advice to heart for some jobs. Sadly, we had too many Hitchhiker fans and :42 became a bit too common.

NDizzle• 11 years ago

Don't run on :17 and :39. Those are mine. Thanks!

pquerna2• 11 years ago

One way to think about your fear is, shouldn't that just be a tarsnap feature?

Add some metadata for a machine that tarsnap should expect a once a day/week/month backup from this machine, and if it doesn't get one, to send you an email?

patio113• 11 years ago

whistles

Until the day when Colin considers it in-scope for Tarsnap, I recommend Deadman's Snitch for this purpose. I literally spend more on DMS to monitor Tarsnap than I spend on Tarsnap. No, I don't think that is just, either.

cperciva18• 11 years ago

the worse-case performance of our daily backup (~150 MB per day delta, ~45 GB total post deduplication) was about 40% longer than our typical case

Yes, that sounds about right. I had maybe half a dozen people write to me who had noticed performance problems, and after the initial "backups failed because the server hit its connection limit" issue, it was people whose backups were already very long-running -- if your daily backups normally take 20 hours to complete, a 40% slowdown is painful.

NeutronBoy• 11 years ago

I run my backups overnight and get a status email each morning, and I didn't even realise there were performance issues until now. As you said, unless you run your backups multiple times per day, or have long-running backups, it may not have had a lot of impact.

FWIW, I live in Australia (so an 'off-peak' timezone), and schedule my cronjob on an odd minute offset, so it may not have been an issue for me anyway!

jldugger2• 11 years ago

We actually have our Chef rdiff backup cookbook randomly distribute jobs across a buckets of time using a hash function of the hostname.

sillysaurus3• 11 years ago

I have to know: Why a hash function of the hostname?

vacri2• 11 years ago

I ran into this recently, backing up munin data to s3. I ran it at a time point offset from an hour to avoid those 'on-the-hour' rushes, but I was getting problems with the copy. Took me a moment to realise I was doing it on a 5-minute boundary, and munin fires on a 5-minute boundary - the data was being updated as I was copying it...

mental note: think harder, next time.

vidarh• 11 years ago

Explicit support for randomizing timers across multiple hosts is a really nice features of the timers provided by systemd:

"AccuracySec=" in *.timer files lets you specify the amount of slack systemd has in firing timers. To quote the documentation "Within this time window, the expiry time will be placed at a host-specific, randomized but stable position that is synchronized between all local timer units."

You may still want to randomize timers locally on a host too, but the above makes automated deployment of timers that affects network services very convenient.

appsonify2• 11 years ago

what the fu....Colin Percival used to be my cello teacher 12 years ago....and he is running tarsnap. My mind is blown.

cperciva18• 11 years ago

Not me -- I'm a violinist, and I've never taught anyone violin either.

Maybe you're thinking of my bother (Graham)? He was teaching cello around that time period I think.

appsonify2• 11 years ago

Oh wow. Yes! Now I remember it was Graham. I was his student around then. Saw your photo on twitter and looked exactly like him! Hahaha. How is Graham?

My mind is just completely blown right now.

ac293• 11 years ago

Sorry if this is offtopic, but can anybody explain the value proposition of tarsnap to me? It seems like a nice service and all, but the pricing is an order of magnitude more expensive than S3. If you are storing a few GB, this might not matter ("over half of Tarsnap users spend under $1 per month on storing their backups"), but if you have that little data, why not just dump it on a free Dropbox/Gdrive/etc account?

For more data, why not just use one of the many compressed, deduplicated, encrypted, incremental backup systems (attic comes to mind, I'm sure there are others) then just sync to S3 at a tenth the cost?

segf4ult• 11 years ago

Because tarsnap is cheap, incredibly well documented, open source, and run by an awesome guy. It's an all around win-win.

stephenr11• 11 years ago

Rsync.net is even cheaper, has no requirement for a custom client, and arguably are more dependable because they're not just reselling S3

Edit: not to mention they offer actual support not just "contact the author" email link as a last resort.

ac293• 11 years ago

But its not cheap, which was my point. 100GB of storage costs:

$300/year at tarsnap

$36/year at S3

pquerna2• 11 years ago

tarsnap is not open source:

"While the Tarsnap code itself has not been released under an open source license, some of the "reusable components" have been published separately under a BSD license"

http://www.tarsnap.com/oss.html

The source code for tarsnap is available to view, so you could audit/inspect it yourself, but it is not under an open source license.

Spooky23• 11 years ago

It depends on the data you are backing up. For dedupe friendly data, the costs come down significantly.

scott_karana2• 11 years ago

Tarsnap can have different write/read keys for each backup archive, so (unlike with Attic) you don't need to worry about a compromised host deleting its entire backup history.

Other than that, Attic is pretty excellent too.

cperciva18• 11 years ago

I suppose I should have known that this would end up at the top of Hacker News...

JacobAldridge2• 11 years ago

It's the picodollars - tarsnap was the second business I fell in love with on HN (the late Kiko was #1) purely because of the awesome vibe I felt emanating from your enterprise (which I'm assuming is a reflection on you as well).

Years later, you've also become a cause celebre for holding true to a clear business and lifestyle vision (again, perceived at distance), in spite of the recommendations and 'support' provided by Patrick and others, including myself. Keep being true, and I suspect the community will keep learning from you Colin.

cperciva18• 11 years ago

It's the picodollars

Hey Thomas, are you listening here?

In all seriousness, the picodollars do an excellent job of attracting exactly the sort of customers I want... and turning away the customers I don't want. They were originally part joke and part a way to avoid arguments with customers who don't understand that 1 GB < 1 GiB, but now it's way more than that.

in spite of the recommendations and 'support' provided by Patrick and others

Don't be too harsh on Patrick. His vision for Tarsnap is not my vision for Tarsnap, but he has helped me to orient myself: The projection of "business" onto the subspace "geek" doesn't look very much like "business", but it's not the same as "kid right out of university who has never had a real job" either, and that's what you would see if I hadn't had advice (from Patrick, Thomas, various YC people, and the rest of HN).

Advice can be very valuable even if you don't follow it to the letter.

jedberg2• 11 years ago

Hey man, awesome writeup. I have a suggestion for you: try and architect off those EBS volumes -- as you unfortunately learned the hard way, they just aren't that consistent. DynamoDB is a good option, or adding some redundancy so that you can just use the ephemeral disk would be even better (and probably cost neutral compared to the "consistent" I/O EBS volumes).

Happy to help if you'd like.

cperciva18• 11 years ago

try and architect off those EBS volumes

Yeah, that has been a work in progress for a long time. FWIW, I started using piops volumes when they were the only SSD option available -- they beat the crap out of spinning ephemeral disks.

Osiris• 11 years ago

For those that want to run a similar service using their own systems, I found that Attic [1] is a great open source backup tool that works in a very similar way, including deduplication and compression.

I backup some VPS servers to my NAS at home using attic over an SSH tunnel. Incremental backups are quite small and it's easy to automate with a simple cron job.

[1] https://attic-backup.org/

middleclick• 11 years ago

How does this compare to Duplicity?

scott_karana2• 11 years ago

It uses git-style addressable blob storage, so you don't have to worry about deltas, because there aren't any.

It's also got more efficient deduplication, because it doesn't use rsync's naïve algorithm.

The downsides: it requires the agent to be remotely installed (a la rsync: no "dumb" backends), and supports less storage backends to boot.

YMMV :-)

k1w1• 11 years ago

As an AWS user this type of thing gives me cause for concern:

At 2015-04-01 00:00 UTC, the Amazon EC2 "provisioned I/O" volume on which most of this metadata was stored suddenly changed from an average latency of 1.2 ms per request to an average latency of 2.2 ms per request. I have no idea why this happened -- indeed, I was so surprised by it that I didn't believe Amazon's monitoring systems at first -- but this immediately resulted in the service being I/O limited.

A sudden doubling of latency can have dire consequences on any system. Knowing that such unexpected changes are possible makes it built trust in your environment, even if it is running fine today.

cperciva18• 11 years ago

Indeed, I didn't know such a change was possible -- that EBS volume went for years with consistent low latency before it suddenly slowed down.

toomuchtodo2• 11 years ago

DevOps/Infrastructure engineer here! I see this happen frequently in AWS. Never expect either your instance networking latency or the latency of the underlying EBS storage layer to be consistent.

If you absolutely need guaranteed IO performance, use an instance store or move to dedicated hardware. Them be the breaks of cloud computing.

http://en.wikipedia.org/wiki/Fallacies_of_distributed_comput...

jeffbarr2• 11 years ago

You could have contacted AWS support or emailed me. Either way, we would have investigated.

MCRed• 11 years ago

It's getting to the point where, when I see a post mortem like this, I am just waiting for the AWS problems. Between this and the downtime that AWS has, I'm kind of amazed that people use it-- you pay too much and you get less. (Compared to a lot of other choices, such as raw metal boxes from Hetzner)

This is why I don't use AWS for anything non-trivial, and I am wary of people who put critical infrastructure on it. (EG: I Don't care about netflix, that service can run on AWS fine, but coinbase, for instance, if I was their customer and they ran on AWS I would stop being their customer.)

Whenever AWS problems come up people talk about how "AWS is so much more efficient, you just outsource that stuff to the experts".

But that seems to imply that hosting on your own hardware in your own office is the only alternative. Of course we stopped doing that in the 1990s.

With AWS you have to know Linux and have ops people, that's true everywhere. With AWS you have the additional burden of learning the AWS APIs and learning how to use AWS, which isn't transferrable, so that's a higher cost. With AWS you have to architect around the limitations of the way AWS is built and your architecture becomes AWS specific if you use those APIS, so that's an additional cost. You don't need any less ops people, probably more, than going with another hosting service like Digital Ocean or Backspace. And if you go with something like Hetzner you pay 1/5th to 1/10th for machines with a lot more performance and local storage. (Though you get the additional latency of being located in Europe, if your primary customers are the USA.)

Of course, I'm also prejudiced. I worked at Amazon and saw how the sausage was made and was not impressed. When AWS was announced as "running on the same infrastructure that powers Amazon.com!!!" as if it was a feature, I cringed. Amazon.com was having outages of parts or major components on a weekly basis at that time. Much of AWS is actually running on bespoke software (so not actually tested by Amazon.com when introduced, though I'm sure portions have been moved over at gunpoint) ... which actually makes it worse. People were trusting their data to a service that pretended to be backing a major e-commerce site but was actually untested outside of the company at the time.

And what have we seen since? An unacceptable level of failures. (in my opinion, of course)

But people seem to be very forgiving. When it's happening everyone's in "how can we fix this mode" and then when it's fixed everyone forgets and goes back to thinking of AWS as always running.

snuxoll• 11 years ago

To this day I still do not get why you would use AWS, the entire user experience is clunky and the pricing is crazy for what you get. Azure isn't much better with regards to downtime, but if you want something more than just a VPS I'd choose it any day over AWS for the significantly better UX in both the admin console and the command line tools + SDK.

Ultimately though, even with Azure or AWS you're going to need people knowledgeable enough to administer your compute instances anyway, so why not just run your full stack on a bunch of VM's from DigitalOcean or Linode or rent a couple dedicated servers and throw oVirt on them; saving yourself a significant chunk of money at the same time.

btmorex4• 11 years ago

Why are you reinventing a scheduler when the OS (at least Linux) already provides a good one?

cperciva18• 11 years ago

I'm talking about scheduling tasks within a single process.

btmorex4• 11 years ago

Threads

Someone• 11 years ago

Good description, but I'm missing lesson learned #0: Do not wait too long before informing your users, even if only to tell them "we know about it and are working on it"

Tarsnap performance issues in late March, most of April

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