At 03:27 PM 8/30/2005, Joshua D. Drake wrote:

If you still have the budget, I would suggest considering either what Ron suggested or possibly using a 4 drive RAID 10 instead.

IME, with only 4 HDs, it's usually better to split them them into two RAID 1's (one for the db, one for everything else including the logs) than it is to put everything on one RAID 10. YMMV.

Really? That's interesting. My experience is different, I assume SCSI? Software/Hardware Raid?

The issue exists regardless of technologies used, although the technology used does affect when things become an irritation or serious problem.

The issue with "everything on the same HD set" seems to be that under light loads anything works reasonably well, but as load increases contention between DB table access, OS access, and xlog writes can cause performance problems.

In particular, _everything_ else hangs while logs are being written with "everything on the same HD set". Thus leaving you with the nasty choices of small log writes that cause more seeking behavior, and the resultant poor overall HD IO performance, or large log writes that basically freeze the server until they are done.

Having the logs on a different HD, and if possible different IO bus, reduces this effect to a minimum and seems to be a better choice than the "shared everything" approach.

Although this effect seems largest when there are fewest HDs, the general pattern is that one should use as many spindles as one can make use of and that they should be as dedicated as possible in their purpose(s). That's why the TPC bench marked systems tend to have literally 100's of HD's and they tend to be split into very focused purposes.

Ron Peacetree

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