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| Newer page: | version 40 | Last edited on Wednesday, June 21, 2006 5:11:16 pm | by GuyThornley | Revert |
| Older page: | version 39 | Last edited on Tuesday, June 20, 2006 12:25:14 am | by GuyThornley | Revert |
@@ -177,13 +177,22 @@
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+!!! Insights and observations
+
+* Using RAID1+0 for the heap files provides a dramatic performance gain. This is because RAID1+0 performs random write very well, compared to RAID5. Using RAID1+0 is an expensive option because of the additional disks (perhaps expensive SCSI disks), space, power and cooling capacity required. Whether the gain is worth the cost is a deployment specific question.
+
+* Current [PostgreSQL] myths claim that moving the [WAL] to its seperate spindles, often on RAID1 or RAID1+0, increases performance. Most of the performance gain arises from increasing the number spindles the total IO load is distributed over, rather than the specific disk configuration. In particular it should be noted that:
+
+ # RAID5, with the help of a battery backed write cache, does sequential write very well.
+ # The [WAL] is written sequentially.
!!! Other observations
-* The WAL consumes large amounts of [Kernel] page cache. When moving the WAL between devices, when the old files are unlinked, 1/2 of the page cache is freed. Since the WAL is never read and written only once, this is as waste!
+* The [
WAL]
consumes large amounts of [Kernel] page cache. When moving the WAL between devices, when the old files are unlinked, 1/2 of the page cache is freed. Since the WAL is never read and written only once, this is as waste!
* The battery-backed write cache makes write performance very erratic.
* The [HP] ~SmartArray hardware (or perhaps driver) tends to block reads while there are cached writes occuring. Large read latencies (whole seconds) result. I have not yet found a way to tune this.
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Part of CategoryDiskNotes
