Bob Friesenhahn wrote:
On Wed, 16 Jun 2010, Arne Jansen wrote:
Please keep in mind I'm talking about a usage as ZIL, not as L2ARC or
main
pool. Because ZIL issues nearly sequential writes, due to the
NVRAM-protection
of the RAID-controller the disk can leave the write cache enabled.
This means
the disk can write essentially with full speed, meaning 150MB/s for a
15k drive.
114000 4k writes/s are 456MB/s, so 3 spindles should do.
Huh? What does the battery backed memory of a RAID-controller have to
do with the unprotected memory of a hard drive? This does not compute.
You're right, I took a wrong turn there. Of course the RAID-controller
disables the write cache of the disks. But because the controller ACKs
each write immediately (as long as it has buffer left), the requests can
be queued in the disk. This enables the disk to write continously.
I double checked before posting: I can nearly saturate a 15k disk if I
make full use of the 32 queue slots giving 137 MB/s or 34k IOPS/s. Times
3 nearly matches the above mentioned 114k IOPS :)
Thanks,
Arne
The flushes that the RAID-controller acks need to be ultimately
delivered to the disk or else there WILL be data loss. The RAID
controller should not purge its own record until the disk reports that
it has flushed its cache. Once the RAID controller's cache is full,
then it should start stalling writes.
Bob
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