Many short writes on ext4 painfully slow -

Martin Langhoff martin.langhoff at
Wed Nov 11 06:19:07 EST 2009

Discussion on fedora-devel on very slow writing to the device when
large files are written in "short" writes. Maybe the kernel isn't
buffering all the writes together, for whatever reason.

ISTR that we've seen some large-file-writing issues on our side as
well -- some related to the SD card firmware, some more likely to be
in the FS stack.

Thread at
---------- Forwarded message ----------
From: Richard W.M. Jones <rjones at>
Date: Wed, Nov 11, 2009 at 11:53 AM
Subject: Re: cpio to ext4 seems much slower than to ext2, ext3 or xfs
To: fedora-devel-list at

On Wed, Nov 11, 2009 at 10:14:21AM +0000, Richard W.M. Jones wrote:
>   echo input | time cpio --quiet -o -H newc > /path/to/fs/output

Update: I found the -C option that lets me specify the blocksize, and
raising it to something sensible (65536) shows major improvements in
performance for all filesystems.

 echo input | time cpio -C 65536 --quiet -o -H newc > /path/to/fs/output

>   tmpfs      0.77 s    x 1.0
>   ext2       1.12 s    x 1.5
>   xfs        1.66 s    x 2.1
>   ext3       2.58 s    x 3.4
>   ext4       5.59 s    x 7.3    <----

The new times are:

 tmpfs     0.20 s    x 1.0
 ext2      0.30 s    x 1.5
 xfs       0.41 s    x 2.1
 ext3      0.57 s    x 2.9
 ext4      0.44 s    x 2.2


Richard Jones, Virtualization Group, Red Hat
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