On Mar 9, 2008, at 20:56, Oliver Martin wrote:
* LVM/md first extent stripe alignment: when creating the PV, specify a --metadatasize that is divisible by all anticipated stripe sizes, i.e., the least common multiple. For example, to accommodate for 3, 4 or 5 drive configurations with 64KB chunk size, that would be 768KB.
Aligning it on chunk-size should be enough so in your case 64KB. Personally I did a lot of tests during the last few weeks and this seemed to make not that big of a difference.
* Alignment of other extents: for the initial array creation with 3 drives the default 4MB extent size is fine. When I add a fourth drive, I can resize the extents with vgchange - though I'm a bit hesitant as the manpage doesn't explicitly say that this doesn't destroy any data. The bigger problem is that the extent size must be a power of two, so the maximum I can use with 192KB stripe size is 64KB. I'll see if that hurts performance. The vgchange manpage says it doesn't...
Why make the extents so small? You do not normally increase your LVs by 4MB. I use 256MB or 512MB extends.
* Telling the file system that the underlying device is striped. ext3 has the stride parameter, and changing it doesn't seem to be possible. XFS might be better, as the swidth/sunit options can be set at mount-time. This would speed up writes, while reads of existing data wouldn't be affected too much by the misalignment anyway. Right?
You can change the stride parameter of ext3 with tune2fs take a look at the -E switch, even after you created the filesystem. That said bonnie++ results showed that while setting a correct stride for EXT3 increased the creation and deletion of files, the big sequential read and write tests suffered. But this is bonnie++......
Hope that helps, Michael
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