Goswin von Brederlow wrote:
Konstantin Sharlaimov <konstantin.sharlaimov@xxxxxxxxx> writes:
On Wed, 2007-11-07 at 10:15 +0100, Goswin von Brederlow wrote:
I wonder if there shouldn't be a way to turn this off (or if there
already is one).
Or more generaly an option to say what is "near". Specifically I would
like to teach the raid1 layer that I have 2 external raid boxes with a
16k chunk size. So read/write within a 16k chunk will be the same disk
but the next 16k are a different disk and "near" doesn't apply
anymore.
Currently there is no way to turn this feature off (this is only a
"request for comments" patch), but I'm planning to make it configurable
via sysfs and module parameters.
Thanks for suggestion for the "near" definition. What do you think about
adding the "chunk_size" parameter (with the default value of 1 chunk = 1
sector). Setting it to 32 will make all reads within 16k chunk to be
considered "near" (with zero distance) so they will go to the same disk.
Max distance will also be configurable (after this distance the "read"
operation is considered "far" and will go to randomly chosen disk)
Regards,
Konstantin
Maybe you need more parameter:
chunk_size - size of a continious chunk on the (multi disk) device
stripe_size - size of a stripe of chunks spanning all disks
rotation_size - size of multiple stripes before parity rotates to a
new disk (sign gives direction of rotation)
near_size - size that is considered to be near on a disk
I would give all sizes in blocks of 512 bytes or bytes.
Why? Would there ever be a time when there would be a significant (or
any) benefit from a size other than a multiple of chunk size? If you
give the rest of the sizes in multiples of chunk size it invites less
human math.
Default would be:
Default would be "zero" to indicate that the raid system should figure
out what to use, allowing the value of "one" to actually mean what it
says. It also invites use of zero for the rest of the calculated sizes,
indicating the raid subsystem should select values. With coming SSD
hardware you may actually want one to mean one.
chunk_size = 1 (block)
stripe_size = 1 (block)
rotation_size = 0 (no rotation)
near_size = 256
That would reflect that you have all chunks continious on a normal
disk and read/writes are done in 128K chunks.
For raid 1 on raid 0:
chunk_size = raid chunk size
stripe_size = num disks * chunk_size
rotation_size = 0
near_size = 256
For raid 1 on raid 5:
chunk_size = raid chunk size
stripe_size = (num disks - 1) * chunk_size
rotation_size = (num disks - 1) * chunk_size (?)
near_size = 256
and so on.
MfG
Goswin
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--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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