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Message-ID: <33c6269f0508162010519a7f12@mail.gmail.com>
Date: Tue, 16 Aug 2005 23:10:38 -0400
From: Alex Turner <armtuk@gmail.com>
To: Anjan Dave <adave@vantage.com>
Subject: Re: choosing RAID level for xlogs
Cc: "Gregory S. Williamson" <gsw@globexplorer.com>,
	pgsql-performance@postgresql.org
In-Reply-To: 
 <4BAFBB6B9CC46F41B2AD7D9F4BBAF785098BE1@vt-pe2550-001.VANTAGE.vantage.com>
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References: 
 <4BAFBB6B9CC46F41B2AD7D9F4BBAF785098BE1@vt-pe2550-001.VANTAGE.vantage.com>

The other point that is well made is that with enough drives you will
max out the PCI bus before you max out the drives.  64-bit 66Mhz can
do about 400MB/sec, which can be acheived by two 3 drive stripes (6
drive in RAID 10).  A true PCI-X card can do better, but can your
controller?  Remember, U320 is only 320MB/channel...

Alex Turner
NetEconomist

On 8/16/05, Anjan Dave <adave@vantage.com> wrote:
> Thanks, everyone. I got some excellent replies, including some long expla=
nations. Appreciate the time you guys took out for the responses.
>=20
> The gist of it i take, is to use RAID10. I have 400MB+ of write cache on =
the controller(s), that the RAID5 LUN(s) could benefit from by filling it u=
p and writing out the complete stripe, but come to think of it, it's shared=
 among the two Storage Processors, all the LUNs, not just the ones holding =
the pg_xlog directory. The other thing (with Clariion) is the write cache m=
irroring. Write isn't signalled complete to the host until the cache conten=
t is mirrored across the other SP (and vice-versa), which is a good thing, =
but this operation could potentially become a bottleneck with very high loa=
d on the SPs.
>=20
> Also, one would have to fully trust the controller/manufacturer's claim o=
n signalling the write completion. And, performance is a priority over the =
drive space lost in RAID10 for me.
>=20
> I can use 4 drives instead of 6.
>=20
> Thanks,
> Anjan
>=20
>         t-----Original Message-----
>         From: Gregory S. Williamson [mailto:gsw@globexplorer.com]
>         Sent: Tue 8/16/2005 6:22 PM
>         To: Anjan Dave; pgsql-performance@postgresql.org
>         Cc:
>         Subject: RE: [PERFORM] choosing RAID level for xlogs
>=20
>=20
>=20
>         I would be very cautious about ever using RAID5, despite manufact=
urers' claims to the contrary. The link below is authored by a very knowled=
gable fellow whose posts I know (and trust) from Informix land.
>=20
>         <http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt>
>=20
>         Greg Williamson
>         DBA
>         GlobeXplorer LLC
>=20
>=20
>         -----Original Message-----
>         From:   pgsql-performance-owner@postgresql.org on behalf of Anjan=
 Dave
>         Sent:   Mon 8/15/2005 1:35 PM
>         To:     pgsql-performance@postgresql.org
>         Cc:
>         Subject:        [PERFORM] choosing RAID level for xlogs
>         Hi,
>=20
>=20
>=20
>         One simple question. For 125 or more checkpoint segments
>         (checkpoint_timeout is 600 seconds, shared_buffers are at 21760 o=
r
>         170MB) on a very busy database, what is more suitable, a separate=
 6 disk
>         RAID5 volume, or a RAID10 volume? Databases will be on separate
>         spindles. Disks are 36GB 15KRPM, 2Gb Fiber Channel. Performance i=
s
>         paramount, but I don't want to use RAID0.
>=20
>=20
>=20
>         PG7.4.7 on RHAS 4.0
>=20
>=20
>=20
>         I can provide more info if needed.
>=20
>=20
>=20
>         Appreciate some recommendations!
>=20
>=20
>=20
>         Thanks,
>=20
>         Anjan
>=20
>=20
>=20
>=20
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>=20
>=20
>=20
>=20
>=20
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>=20
>=20
>=20
>=20
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