Re: Linux 2.6.29

[Ric Wheeler]

Linus Torvalds wrote:
> On Mon, 30 Mar 2009, Ric Wheeler wrote:
> > People keep forgetting that storage (even on your commodity s-ata class of
> > drives) has very large & volatile cache. The disk firmware can hold writes in
> > that cache as long as it wants, reorder its writes into anything that makes
> > sense and has no explicit ordering promises.
>
> Well, when it comes to disk caches, it really does make sense to start 
> looking at what breaks.
>
> For example, it is obviously true that any half-way modern disk has 
> megabytes of caches, and write caching is quite often enabled by default. 
>
> BUT!
>
> The write-caches on disk are rather different in many very fundamental 
> ways from the kernel write caches.
>
> One of the differences is that no disk I've ever heard of does write- 
> caching for long times, unless it has battery back-up. Yes, yes, you can 
> probably find firmware that has some odd starvation issue, and if the disk 
> is constantly busy and the access patterns are _just_ right the writes can 
> take a long time, but realistically we're talking delaying and re-ordering 
> things by milliseconds. We're not talking seconds or tens of seconds.
>
> And that's really quite a _big_ difference in itself. It may not be 
> qualitatively all that different (re-ordering is re-ordering, delays are 
> delays), but IN PRACTICE there's an absolutely huge difference between 
> delaying and re-ordering writes over milliseconds and doing so over 30s.
>
> The other (huge) difference is that the on-disk write caching generally 
> fails only if the drive power fails. Yes, there's a software component to 
> it (buggy firmware), but you can really approximate the whole "disk write 
> caches didn't get flushed" with "powerfail".
>
> Kernel data caches? Let's be honest. The kernel can fail for a thousand 
> different reasons, including very much _any_ component failing, rather 
> than just the power supply. But also obviously including bugs.
>
> So when people bring up on-disk caching, it really is a totally different 
> thing from the kernel delaying writes.
>
> So it's entirely reasonable to say "leave the disk doing write caching, 
> and don't force flushing", while still saying "the kernel should order the 
> writes it does".

Largely correct above - most disks will gradually destage writes from their 
cache. Large, sequential writes might entirely bypass the write cache and be 
sent (more or less) immediately out to permanent storage.

I still disagree strongly with the don't force flush idea - we have an absolute 
and critical need to have ordered writes that will survive a power failure for 
any file system that is built on transactions (or data base).

The big issues are that for s-ata drives, our flush mechanism is really, really 
primitive and brutal. We could/should try to validate a better and less onerous 
mechanism (with ordering tags? experimental flush ranges? etc).

> Thinking that this is somehow a black-and-white issue where "ordered 
> writes" always has to imply "cache flush commands" is simply wrong. It is 
> _not_ that black-and-white, and it should probably not even be a 
> filesystem decision to make (it's a "system" decision).
>
> This, btw, is doubly true simply because if the disk really fails, it's 
> entirely possible that it fails in a really nasty way. As in "not only did 
> it not write the sector, but the whole track is now totally unreadable 
> because power failed while the write head was active".

I spent a very long time looking at huge numbers of installed systems (millions 
of file systems deployed in the field), including  taking part in weekly 
analysis of why things failed, whether the rates of failure went up or down with 
a given configuration, etc. so I can fully appreciate all of the ways drives (or 
SSD's!) can magically eat your data.

What you have to keep in mind is the order of magnitude of various buckets of 
failures - software crashes/code bugs tend to dominate, followed by drive 
failures, followed by power supplies, etc.

I have personally seen a huge reduction in the "software" rate of failures when 
you get the write barriers (forced write cache flushing) working properly with a 
very large installed base, tested over many years :-)


> Because that notion of "power" is not a digital thing - you have 
> capacitors, brown-outs, and generally nasty "oops, for a few milliseconds 
> the drive still had power, but it was way out of spec, and odd things 
> happened".
>
> So quite frankly, if you start worrying about disk power failures, you 
> should also then worry about the disk failing in _way_ more spectacular 
> ways than just the simple "wrote or wrote not - that is the question".

Again, you have to focus on the errors that happen in order of the prevalence.

The number of boxes, over a 3 year period, that have an unexpected power loss is 
much, much higher than the number of boxes that have a disk head crash (probably 
the number one cause of hard disk failure).

I do agree that we need to do other (background) tasks to detect things like the 
 that drives can have (lots of neat terms that give file system people 
nightmare in the drive industry: "adjacent track erasures", "over powered 
seeks", "hi fly writes" just to name my favourites).

Having full checksumming for data blocks and metadata blocks in btrfs will allow 
us to do this kind of background scrubbing pretty naturally, a big win.

> And when was the last time you saw a "safe" logging filesystem that was 
> safe in the face of the log returning IO errors after power comes back on?

This is pretty much a double failure - you need a bad write to the log (or 
undetected media error like the ones I mentioned above) and a power failure/reboot.

As you say, most file systems or data bases will need manual repair or will get 
restored from tape.

That is not the normal case, but we can do surface level scans to try and weed 
out bad media continually during the healthy phase of a boxes life. This can be 
relatively low impact and has a huge positive impact on system reliability.

Any engineer who designs storage system knows that you will have failures - we 
just aim to get the rate of failures down to where you have a fighting chance of 
recovery at a price you can afford...

> Sure, RAID is one answer. Except not so much in 99% of all desktops or 
> especially laptops.
>
> 			Linus

If you only have one disk, you clearly need a good back up plan of some kind. I 
try to treat my laptop as a carrying vessel for data that I have temporarily on 
it, but is stored somewhere else more stable for when the disk breaks, some kid 
steals it, etc :-)

Ric

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