User-Agent: Microsoft-Entourage/11.3.3.061214
Date: Wed, 30 May 2007 13:13:45 -0700
Subject: Re: Bad RAID1 read performance
From: "Luke Lonergan" <llonergan@greenplum.com>
To: "Albert Cervera Areny" <albert@sedifa.com>,
	pgsql-performance@postgresql.org
Message-ID: <C2832909.31C9D%llonergan@greenplum.com>
Thread-Topic: [PERFORM] Bad RAID1 read performance
Thread-Index: Acei9wZRRRiMMg7qEdy5oAAX8s8Cag==
In-Reply-To: <200705301700.26704.albert@sedifa.com>
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Albert,

On 5/30/07 8:00 AM, "Albert Cervera Areny" <albert@sedifa.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@sedifa.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?
>>> 
>>> 
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