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

[Zurell, Falko]

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 at redhat.com <linux-lvm-bounces at redhat.com>
An: LVM general discussion and development <linux-lvm at 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 at 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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