Date: Thu, 28 Nov 2013 21:47:17 +0100 From: =?ISO-8859-1?Q?Szalai_Andr=E1s?= <szalai.bandi@gmail.com> To: freebsd-stable@freebsd.org Subject: gmirror: writes are faster than reads Message-ID: <5297ABD5.5060504@gmail.com>
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Hi Guys,
Has somebody encountered (significantly) different read/write speeds
when using gmirror?
I have 2xWD WD30EFRX RED drives which are configured as follows:
$ gmirror status
Name Status Components
mirror/root COMPLETE ada0p2 (ACTIVE)
ada1p2 (ACTIVE)
mirror/data COMPLETE ada0p4 (ACTIVE)
ada1p4 (ACTIVE)
mirror/root is mounted as the root fs (UFS2).
Doing write:
$ time dd if=/dev/zero of=/IMAGE bs=1024k count=`expr 4 \* 1024`
4096+0 records in
4096+0 records out
4294967296 bytes transferred in 29.326044 secs (146455733 bytes/sec)
Doing read:
$ time dd if=/IMAGE of=/dev/null bs=1024k count=`expr 4 \* 1024`
4096+0 records in
4096+0 records out
4294967296 bytes transferred in 48.821649 secs (87972598 bytes/sec)
As you can see, read is much slower than write (87 vs 146 MB/s). Why?
Any help would be appreciated.
Best regards,
Andrew
PS: Partition layout (partitions are 4k aligned):
$ gpart show ada0 ada1
=> 34 5860533101 ada0 GPT (2.7T)
34 6 - free - (3.0k)
40 1024 1 freebsd-boot (512k)
1064 16777216 2 freebsd-ufs (8.0G)
16778280 16777216 3 freebsd-swap (8.0G)
33555496 5826977632 4 freebsd-ufs (2.7T)
5860533128 7 - free - (3.5k)
=> 34 5860533101 ada1 GPT (2.7T)
34 6 - free - (3.0k)
40 1024 1 freebsd-boot (512k)
1064 16777216 2 freebsd-ufs (8.0G)
16778280 16777216 3 freebsd-swap (8.0G)
33555496 5826977632 4 freebsd-ufs (2.7T)
5860533128 7 - free - (3.5k)
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