Very low disk performance on 5.x
Poul-Henning Kamp
phk at phk.freebsd.dk
Mon May 2 06:23:01 PDT 2005
In message <4276281C.6060209 at centtech.com>, Eric Anderson writes:
>Poul-Henning Kamp wrote:
>> In message <427626DC.5030702 at centtech.com>, Eric Anderson writes:
>>
>>
>>>Don't mean to be terse here, but I'm talking about the same test done an
>>>two different RAID5 configurations, with different disks, and not just
>>>me - other users in this very thread see the same issue..
>>
>>
>> Uhm, if you are using RAID5 and your requests are not aligned and
>> sized after the RAID5 you should *expect* read performance to be poor.
>>
>> If you your request ends up accessing two different blocks even just
>> once per stripe, this totally kills performance.
>
>Wouldn't this be a problem for writes then too?
I presume you would only compare read to write performance on a RAID5
device which has battery backed cache.
Without a battery backed cache (or pretending to have one) RAID5
write performance is abysmall no matter which alignment you use.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
More information about the freebsd-performance
mailing list