linux-ext4 - Re: HUGE slowdown when doing dpkg with ext4 over nbd

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Date:   Wed, 7 Dec 2016 10:58:25 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Renaud Mariana <rmariana@...ine.net>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: HUGE slowdown when doing dpkg with ext4 over nbd

On Dec 7, 2016, at 2:52 AM, Renaud Mariana <rmariana@...ine.net> wrote:
> 
> Here are my answers, hope it will help solve this issue, thanks.
> 
> Recap:
> dpkg kibana on ext4 over a nbd device takes 10 minutes
> with xfs it's only 30s.
> with ext4 no extends only 30s.
> 
> 
> kernels :
> 4.5.7 has this issue as older kernel like 4.4.34
> The issue is also when nbd client & server run on same host
> 
> 
> How small are the files?
> here is the histogram of file sizes : http://pasteboard.co/6HC3nKyk2.png
> We can see 5000 files around 512 Bytes.

Definitely there is no value to use fallocate for 512-byte files, or any
of the files that can be written in a single write() syscall.  I'd expect
any reasonable tool to be using a write buffer of at least 2-4MB these
days to get good performance, so writes below the buffer size shouldn't
use fallocate() at all.

> dpkg using fallocate() ?
> Yes, there are 16044 calls by the same process
> what are these uninitialized extents ?

Uninitialized extents are preallocated ranges of a file on disk that will
read back as zero, but are not necessarily zero-filled at allocation time.
For large files that are written randomly (or written slowly and may have
contention from other writers) fallocate() + uninitialized extents will
preallocate the space for the file so that it is (largely) contiguous on
disk and overwrites will not result in random block allocation.

Cheers, Andreas

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