Re: enhancements to the raid1 driver

On Friday February 14, Paul.Clements@SteelEye.com wrote:

Hi,

I'm just beginning a project to add some new features to the raid1
driver. The planned features include:

intent log - a disk-backed bitmap to allow for quick resyncs (Peter T.
Breuer's fr1 code looks very promising)

An intent log would be just as useful for raid5 as for raid1, so I
think it should be largely supported by md.c rather than specific to
raid1.c

asynchronous replication - to get acceptable performance when the backup
leg of the raid1 mirror is slow (e.g., nbd over a LAN or WAN)

Hmmm. I cannot figure out exactly what would be asynchronous...  Nor
can I see how this would require an ioctl.

In general, ioctl's are seen a evils to be avoided, not added, so I am
not in favour of the patch you suggest.

Maybe if you explain more about what would be the specifics of these
ioctls that you want to pass down.

NeilBrown

Neil,

Neil Brown wrote:

Maybe if you explain more about what would be the specifics of these
ioctls that you want to pass down.

Sure, no problem...
 

quoted

asynchronous replication - to get acceptable performance when the backup
leg of the raid1 mirror is slow (e.g., nbd over a LAN or WAN)

Hmmm. I cannot figure out exactly what would be asynchronous...  Nor
can I see how this would require an ioctl.

Basically, we're trying to overcome latency penalties associated with
writing data and waiting for acknowledgements over a network (or any
other slow transport). So, all writes to the backup device(s) would be
queued, so that the writes could take place asynchronously. The size of
the queue and whether or not to enable this asynchronous mode would be
user-configurable; that's where the ioctls come in. Since user
configuration of the md driver is currently done with ioctls, this
seemed like the natural choice. Of course, I'm open to other
suggestions, if there's a better way to do this...

--
Paul