On Thursday, 23 March 2000 at 17:44:38 -0600, Dan Nelson wrote:
> In the last episode (Mar 23), Greg Lehey said:
>>
>> Agreed. This is on the Vinum wishlist, but it comes at the expense of
>> reliability (how long do you wait to cluster? What happens if the
>> system fails in between?). In addition, for Vinum it needs to be done
>> before entering the hardware driver.
>
> For the simplest case, you can choose to optimize only when the user
> sends a single huge write().
We discussed that. Since the optimum band size is much larger than
MAXPHYS, this can't happen on a correctly configured system.
> That way you don't have to worry about caching dirty pages in vinum.
> This is basically what the hardware RAIDs I have do.
Right, but that seriously degrades normal non-band writes.
> They'll only do the write optimization (they call it "pipelining")
> if you actually send a single SCSI write request large enough to
> span all the disks. I don't know what would be required to get our
> kernel to even be able to write blocks this big (what's the upper
> limit on MAXPHYS)?
MAXPHYS is currently 128 kB. I recommend stripes of 256 kB to 512 kB,
so with a 9 disk RAID we're talking about bands of 2 to 4 MB. My
current idea is to set a flag on each volume specifying that it's
prepared to wait up to n seconds for write clustering.
Greg
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