Re: Bad RAID1 read performance

Albert,

On 5/30/07 8:00 AM, "Albert Cervera Areny" <albert(at)sedifa(dot)com> wrote:

> Hardware isn't very good I believe, and it's about 2-3 years old, but the RAID
> is Linux software, and though not very good the difference between reading
> and writing should probably be greater... (?)

Not for one thread/process of I/O. Mirror sets can nearly double the read
performance on most RAID adapters or SW RAID when using two or more
thread/processes, but a single thread will get one drive worth of
performance.

You should try running two simultaneous processes during reading and see
what you get.

> Would you set 512Kb readahead on both drives and RAID? I tried various
> configurations and none seemed to make a big difference. It seemed correct to
> me to set 512kb per drive and 1024kb for md0.

Shouldn't matter that much, but yes, each drive getting half the readahead
is a good strategy. Try 256+256 and 512.

The problem you have is likely not related to the readahead though - I
suggest you try read/write to a single disk and see what you get. You
should get around 60 MB/s if the drive is a modern 7200 RPM SATA disk. If
you aren't getting that on a single drive, there's something wrong with the
SATA driver or the drive(s).

- Luke
> A Dimecres 30 Maig 2007 16:09, Luke Lonergan va escriure:
>> This sounds like a bad RAID controller - are you using a built-in hardware
>> RAID? If so, you will likely want to use Linux software RAID instead.
>>
>> Also - you might want to try a 512KB readahead - I've found that is optimal
>> for RAID1 on some RAID controllers.
>>
>> - Luke
>>
>> On 5/30/07 2:35 AM, "Albert Cervera Areny" <albert(at)sedifa(dot)com> wrote:
>>> Hi,
>>> after doing the "dd" tests for a server we have at work I obtained:
>>> Read: 47.20 Mb/s
>>> Write: 39.82 Mb/s
>>> Some days ago read performance was around 20Mb/s due to no readahead in
>>> md0 so I modified it using hdparm. However, it seems to me that being it
>>> a RAID1 read speed could be much better. These are SATA disks with 3Gb of
>>> RAM so I did 'time bash -c "dd if=/dev/zero of=bigfile bs=8k count=786432
>>> && sync"'. File system is ext3 (if read many times in the list that XFS
>>> is faster), but I don't want to change the file system right now.
>>> Modifing the readahead from the current 1024k to 2048k doesn't make any
>>> difference. Are there any other tweaks I can make?
>>>
>>>
>>> ---------------------------(end of broadcast)---------------------------
>>> TIP 4: Have you searched our list archives?
>>>
>>> http://archives.postgresql.org
>>
>> ---------------------------(end of broadcast)---------------------------
>> TIP 6: explain analyze is your friend