From: Christoph Hellwig <hch@infradead.org>
To: Renaud Mariana <rmariana@online.net>
Cc: Andreas Dilger <adilger@dilger.ca>, linux-ext4@vger.kernel.org
Subject: Re: HUGE slowdown when doing dpkg with ext4 over nbd
Date: Wed, 7 Dec 2016 08:24:22 -0800 [thread overview]
Message-ID: <20161207162422.GA11436@infradead.org> (raw)
In-Reply-To: <1339224178.8897979.1481104368542.JavaMail.zimbra@online.net>
We had a pretty similar issue lately with SCSI, and I suspect it's
something similar. There are two interrelated issues:
- dpkg uses fallocate where it absolutely shouldn't - it creates
new files with typically a single write call, so fallocate doesn't
help anything, but actively hurts.
- For small enough regions ext4 does not create unwritten extents
but zeroes data on disk, and whenever zeroing out the data is
expensive this really shows up. The SCSI case was a device
that has a horrible slow WRITE_SAME implementation, but for nbd
we'll just write the zero page repeatedly.
Patching out the stupid fallocate in dpkg will speed ext4 up a lot
(especially for small files) and will probably speed XFS up a tiny
bit as well. But all my mails to the dpkg folks were simply ignored
unfortunately.
next prev parent reply other threads:[~2016-12-07 16:24 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-06 13:13 HUGE slowdown when doing dpkg with ext4 over nbd Renaud Mariana
2016-12-06 18:45 ` Andreas Dilger
2016-12-07 9:52 ` Renaud Mariana
2016-12-07 16:24 ` Christoph Hellwig [this message]
2016-12-07 17:58 ` Andreas Dilger
2016-12-07 18:12 ` Andreas Dilger
2016-12-07 18:16 ` Andreas Dilger
2016-12-07 18:34 ` Sven Joachim
2016-12-07 20:14 ` Andreas Dilger
2016-12-08 13:14 ` Renaud Mariana
2016-12-09 1:25 ` Dave Chinner
2016-12-09 20:28 ` Andreas Dilger
2016-12-09 21:31 ` Dave Chinner
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