AW: Re: [linux-lvm] very slow fw performance when snapshots active

AW: Re: [linux-lvm] very slow fw performance when snapshots active

[Zurell, Falko]

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Ok, i understand the point with the swap file of vi. But why is this with snapshots activated that much slower than without? 
Why can the file be saved in seconds without cow instead of about 2 minutes with cow. Needing twice the time for saving would be reasonable.

Editing with vi was just a performance test ;-)

Thanks and kind regards


Falko Zurell

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----- Originalnachricht -----
Von: linux-lvm-bounces@redhat.com <linux-lvm-bounces@redhat.com>
An: LVM general discussion and development <linux-lvm@redhat.com>
Gesendet: Thu Oct 18 15:52:36 2007
Betreff: Re: [linux-lvm] very slow fw performance when snapshots active

Hi,

On 10/18/2007 10:39:50 AM +0200, Falko Zurell <falko.zurell@idmedia.com> 
wrote:
> 
> Then I edit a 2 GB text file on /opt (XFS filesystem) with vi (just
> hitting "enter" and save this minor change). This will take minutes to
> finish the vi during saving.

This problem is not lvm-related: when opening a file with vim, it reads 
the entire file into memory (even if the file is 2GB), marks the changes 
into a .swp file. When saving, vim rewrites the entire file in place, 
effectively changing 2GB of copy on write blocks. This happens every 
time, with or without snapshot, the latter being worse because of the 
cow overhead.

for reading-only very large files, "less" or "more" read files on-demand 
which is a far more effecient way.

for writing, I don't know any method allowing to safely write a small 
change into a large file, except using awk or perl parsing (which reads 
the entire streamed file anyway)

Gabriel

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Re: AW: Re: [linux-lvm] very slow fw performance when snapshots active

[Gabriel Barazer]

On 10/18/2007 4:01:40 PM +0200, "Zurell, Falko" 
<Falko.Zurell@idmedia.com> wrote:
> Ok, i understand the point with the swap file of vi. But why is this 
> with snapshots activated that much slower than without?
> Why can the file be saved in seconds without cow instead of about 2 
> minutes with cow. Needing twice the time for saving would be reasonable.

I think this is because the re-writing operation on a standard block 
device is done using a comparison between the data to write, and the 
data already written (which is nearly all the time read from the OS file 
cache in your test case, instead of the block device itself, because the 
file written was entirely read just before) whereas the LVM snapshot 
target redirect any write to the COW space without this comparison step. 
Maybe there is a possible optimization of the snapshot dm target which 
could compare block before deciding to write them on the COW space. 
Obvisouly, such optimization process add a slight overhead an can 
possibly have a negative impact on the performance.

Gabriel

Re: [linux-lvm] very slow fw performance when snapshots active

[malahal]

Reducing chunksize may help a bit but you may end up having
performance problems when you read snapshot data later.

Thanks, Malahal.

Gabriel Barazer [gabriel@oxeva.fr] wrote:
> On 10/18/2007 4:01:40 PM +0200, "Zurell, Falko" 
> <Falko.Zurell@idmedia.com> wrote:
> >Ok, i understand the point with the swap file of vi. But why is this 
> >with snapshots activated that much slower than without?
> >Why can the file be saved in seconds without cow instead of about 2 
> >minutes with cow. Needing twice the time for saving would be reasonable.
> 
> I think this is because the re-writing operation on a standard block 
> device is done using a comparison between the data to write, and the 
> data already written (which is nearly all the time read from the OS file 
> cache in your test case, instead of the block device itself, because the 
> file written was entirely read just before) whereas the LVM snapshot 
> target redirect any write to the COW space without this comparison step. 
> Maybe there is a possible optimization of the snapshot dm target which 
> could compare block before deciding to write them on the COW space. 
> Obvisouly, such optimization process add a slight overhead an can 
> possibly have a negative impact on the performance.
> 
> Gabriel
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/