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Re: [lvm-devel] [PATCH v1 00/30] Ext4 snapshots

On Mon, Jun 13, 2011 at 1:54 PM, Lukas Czerner <lczerner redhat com> wrote:
> On Mon, 13 Jun 2011, Amir G. wrote:
>> On Fri, Jun 10, 2011 at 12:00 PM, Lukas Czerner <lczerner redhat com> wrote:
>> >
>> > --snip--
>> >
>> > Hi Amir,
>> >
>> > that is why I spoke with several dm people and all of them had the same
>> > opinion. When you are not using the advantage of being at fs level,
>> > there is no reason to have shapshoting at this level.
>> >
>> > And no, I am not blinded. I am trying to understand why is multisnap a
>> > huge win everyone is saying, so I already asked ejt to step in and
>> > give us an overview on how dm-multisnap works and why is it better
>> > than the old implementation. Also I am trying it myslef, and so far
>> > it works quite well. I might have some numbers later.
>> >
>> (Dropping LKML - had enough of that attention for 1 week...)
>> Hi Lukas,
>> So did you get any numbers? Joe said you were not able to get good results.
> Hi, yes I did had some bad numbers, but it was due to stupid setup I
> have created :) metadata and data volume on the same drive, but in the
> different partition. In the postmark test the performance drop was about
> 100% and that is quite expected as it probably caused a LOT of seeks.
> But when I separated data and metadata I have very good results. Results
> differs with the data block size used by dm.
> Filesystem on bare device.
> 113 76 657.89 661.60 2000.00 325.80 328.61 329.28 661.60 1980.88 332.09
> 24363052.00 76242168.00
> dm-multisnap
> bs=128
> 146 118 423.73 512.06 1923.08 209.84 211.64 212.08 512.06 1904.69 213.89
> 18856334.00 59009348.00
> bs=256
> 151 96 520.83 495.11 943.40 257.93 260.15 260.68 495.11 934.38 262.91
> 18231952.00 57055396.00
> bs=512
> 134 96 520.83 557.92 1515.15 257.93 260.15 260.68 557.92 1500.67 262.91
> 20544960.00 64293764.00
> bs=1024
> 119 70 714.29 628.24 1470.59 353.73 356.77 357.50 628.24 1456.53 360.56
> 23134662.00 72398024.00
> bs=2048
> 128 76 657.89 584.07 1190.48 325.80 328.61 329.28 584.07 1179.10 332.09
> 21508006.00 67307536.00
> bs=4096
> 131 84 595.24 570.69 1851.85 294.77 297.31 297.92 570.69 1834.15 300.46
> 21015456.00 65766144.00
> Legend:
> -----------------------------------------------------------------------
> Total_duration Duration_of_transactions Transactions/s
> Files_created/s Creation_alone/s Creation_mixed_with_transaction/s
> Read/s Append/s Deleted/s Deletion_alone/s
> Deletion_mixed_with_transaction/s Read_B/s Write_B/s
> -----------------------------------------------------------------------
> I choosed postmark because it is doing a lot of operation on the file
> and it is quite metadata intensive. Although it is still very simple
> and limited test. However you can see that with data block size 1024B I
> received almost the same results as in the case of bare device. It means
> that there was almost none performance drop and I suspect that if I put
> metadata on the SSD it would not be noticeable at all.
> We can see that results are dropping to the bs of 1024B and rising
> afterwards. I suspect that we are dealing with two variables with
> opposite outcome. Thinp target works better with bigger block sizes as
> it has less metadata to work with, but in the other hand snapshots are
> then more expensive, because we have to deal with COW rather than simple
> write when we are changing the whole block. But 1024 seems quite
> reasonable and I also think that putting metadata on SSD (which is
> easily doable) we can very well address the first one.

SSD may be doable for enterprise servers, but I don't have one in my laptop :-(

I think that Joe will agree with me that this is not the benchmark he
is concerned about.
It is clear to me that any operations applied to a new thin
provisioned file system, will
perform well, sometimes even better than on bare device.

Did you use subdirs in postmark?
If you do, ext3 will try to spread subdirs under root all over the disk
(not sure about ext4) and postmark will be slower on bare device.

The benchmark which is relevant to the drawbacks of multisnap is aging
a filesystem
to the point that it's metadata is physically layed out very
differently than was intended.

Here is a suggested real life test:

1. DATE=START_DATE; take snapshot $DATE
2. git checkout <mainline daily git tag>
3. time -o LOGFILE --append make
4. DATE+=DAY; goto 1

Repeat this test until the volume fills up to several orders of
magnitude more than the size
of RAM on your system and observe how build time changes over time.

>> Did you come to understand the drawbacks of multisnap (physical fragmentation)?
> Yes I did, but the fragmentation is problem for any thinly provisioned
> storage. I also understand that your snapshot files has also proble with
> fragmentation.

It's true. ext4 snapshots generates fragmented *files*, but it does not fragment
the filesystem metadata. And only on specific workloads of in-place writes,
like large db or virtual image.

One difference is that ext4 snapshots can do effective auto defrag by using
the inode context, which is not available for multisnap.
The other big difference is that ext4 snapshots gives precedence to main
fs performance, while multisnap hasn't even the notion of a main fs.
All thinp and snapshot targets are writable and get equal treatment.

>> Did it make you change your mind about ext4 snapshots?
> From the first time I was interested in ext4 snapshgots, however as I
> came to understand how it works (I must admit not *very* deeply) it all
> seems like a hack to solve your problem at the time (several years
> ago).

The problem was not mine, it's was for all Linux users who wanted snapshots.
The future does look brighter for them, but CTERA customers don't have to wait
for the future...

> And now, when I see how the new dm-multisnap target works, what features it has,
> how it performs (more-or-less) it seems to me that it is a lot more
> flexible and desirable way of doing this.
> On the other hand your snapshots disrupts a quite calm water of
> stable filesystem with a very poor set of features and very limited
> possibilities of improvements. Not talking about maintaining burden. But
> yes, it might perform a bit better.
> So to sum it up I see that dm-multisnap has superset of features your
> ext4 snapshots has, in performs well enough, it is more generic solution
> for all filesystems, it is also more flexibile, it does not require
> intrusive change into stable fs code, and it has better possibilities of
> future improvements.
> Do even if the final decision does not belong to me, I think that we do
> not need this code in ext4. If your snapshots were a *real* filesystem level
> snapshots with all the cool features it provides, the situation would be
> quite different, however even then I would be thinking if it is worth
> it, when we have btrfs here and now, ready to use, and improving every
> day to get at enterprise level (it will, hopefully, be a default
> filesystem in Fedora 16, which is huge step forward to enterprise
> environment).
> And here I would very much like to see other ext4 developers opinions,
> because they were really quiet on this matter and it is time to reveal
> the cards on the table, so ?...
>> I am planning to join the ext4 weekly call today and ask if people think that
>> we still have open issues with ext4 snapshots, which must be resolved
>> before the merge.
>> I have 2 questions that should be answered before the merge:
>> 1. Should 32bit ext4 move to 48bit snapshot file format after the
>> format is implemented for 64bit ext4?
>> 2. Should exclude bitmap be allocated only on mkfs time or should it
>> also be possible to allocate it with tune2fs?
>> Allocating it later will enable snapshots on existing fs, but will
>> have sub-optimal on-disk layout.
>> If anyone has opinions on these 2 questions, please make them heard here or on
>> the call today.
>> Thanks,
>> Amir.
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