Re: BTRFS: Unbelievably slow with kvm/qemu

[K. Richard Pixley]

 On 9/2/10 09:36 , K. Richard Pixley wrote:
>  On 9/1/10 17:18 , Ted Ts'o wrote:
> > On Tue, Aug 31, 2010 at 02:58:44PM -0700, K. Richard Pixley wrote:
> > >   On 20100831 14:46, Mike Fedyk wrote:
> > > > There is little reason not to use duplicate metadata.  Only small
> > > > files (less than 2kb) get stored in the tree, so there should be no
> > > > worries about images being duplicated without data duplication set at
> > > > mkfs time.
> > > My benchmarks show that for my kinds of data, btrfs is somewhat
> > > slower than ext4, (which is slightly slower than ext3 which is
> > > somewhat slower than ext2), when using the defaults, (ie, duplicate
> > > metadata).
> > >
> > > It's a hair faster than ext2, (the fastest of the ext family), when
> > > using singleton metadata.  And ext2 isn't even crash resistant while
> > > btrfs has snapshots.
> > I'm really, really curious.  Can you describe your data and your
> > workload in detail?  You mentioned "continuous builders"; is this some
> > kind of tinderbox setup?
> I'm not familiar with tinderbox.  Continuous builders tend to be a lot 
> like shell scripts - its usually easier to write a new one than to 
> even bother to read someone else's.  :).
>
> Basically, it's an automated system that started out life as a shell 
> script loop around a build a few years ago.  The current rendition 
> includes a number of extra features.  The basic idea here is to expose 
> top-of-tree build errors as fast as possible which means that these 
> machines can take some build shortcuts that would not be appropriate 
> for official builds intended as release candidates.  We have a 
> different set of builders which build release candidates.
>
> When it starts, it removes as many snapshots as it needs to in order 
> to make space for another build.  Initially it creates a snapshot from 
> /home, checks out source, and does a full build of top of tree.  Then 
> it starts over.  If it has a build and is not top of tree, it creates 
> a snapshot from the last successful build, updates, and does an 
> incremental build.  When it reaches top of tree, it starts taking 
> requests.
>
> We're using openembedded so the build is largely based on components 
> with a global "BOM", (bill of materials), acting as a code based 
> database of which versions of which components are in use for which 
> images.  This acts as a funneling point.  Requests are a specification 
> of a list of components to change, (different versions, etc).  A 
> snapshot is taken from the last successful build, the BOM is changed 
> locally and built incrementally.  If everything builds alright, then 
> the new BOM may be committed and/or the resulting binary packages may 
> be published for QA consumption.  But even in the case of failure, 
> this snapshot is terminal and never marked as "successful" so never 
> reused.
>
> The system acts both as a continuous builder to check top of tree as 
> well as an automated method for serializing changes, (which stands in 
> for real, human integration).
>
> We currently have about 20 of these servers, ranging from 2 - 24 
> cores, 4 - 24G memory, etc.  A single device build takes about 22G so 
> a 24G machine can do an entire build in memory.  The different 
> machines run similar builds against different branches or against 
> different targets and the staggering tends to create a lower response 
> time in the case of top-of-tree build errors that affect all devices, 
> (the most common type of error).  And most of the servers are cast 
> offs, older servers that would be discarded otherwise.  Server speed 
> tends to be an issue primarily for the full builds.  Once the full 
> build has been created, the incrementals tend to be limited to single 
> threading as the build spends most of it's time doing dependency 
> rechecking.
>
> The snapshot based approach is recent, as is our btrfs usage, (which 
> is currently problematic, polluted file systems, kernel crashes, 
> etc).  Previously I was using rsync to backup a copy of a full build 
> and rsync to replace it when a build failed.  The working directory 
> was the same working directory and I went to some pains to make it 
> reusable.  I've been looking for a snapshotting facility for a couple 
> of years now but only discovered btrfs recently.  (I tried lvm based 
> snapshots but they don't really have the characteristics that I want, 
> nor do nilfs2 snapshots.)
>
> Is that what you were looking for?
I should probably mention times and targets.

A typical 2-core, 4G developer workstation can build our entire system 
for 1 device in about 6 - 8hrs.  We typically build each device on a 
separate server and the highest end servers we're using today, (8 - 24 
core, 24G memory), can build a single device in a little under an hour.  
Those are full build times.  A complete cycle of an incremental based 
builder, (doing nothing but bookkeeping and checking dependencies), can 
take anywhere from about 2 - 4 minutes.  And a typical single component 
update usually takes 4 - 6 minutes.

From a developer's perspective, I'm churning out 8hr builds every 5 
minutes or so.  What snapshots provide primarily is the ability to 
discard a polluted/broken working directory while retaining the ability 
to reuse it's immediate predecessor.  It's also true that snapshots 
leave old working directories laying around where they could be examined 
or debugged, but generally that facility is rarely used because it's too 
much trouble to provide developers access to those machines.

The targets here are an openembedded based embedded linux system.

--rich
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