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Re: [Xen-users] Cheap IOMMU hardware and ECC support importance

  • To: xen-users@xxxxxxxxxxxxx
  • From: Gordan Bobic <gordan@xxxxxxxxxx>
  • Date: Sat, 28 Jun 2014 13:45:01 +0100
  • Delivery-date: Sat, 28 Jun 2014 12:45:31 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

On 06/28/2014 01:23 PM, Kuba wrote:
W dniu 2014-06-28 13:25, lee pisze:
Kuba <kuba.0000@xxxxx> writes:

W dniu 2014-06-28 09:45, lee pisze:

I don't know about ZFS, though, never used that.  How much CPU overhead
is involved with that?  I don't need any more CPU overhead like comes
with software raid.

ZFS offers you two things RAID controller AFAIK cannot do for you:
end-to-end data checksumming and SSD caching.

There might be RAID controllers that can do SSD caching.  SSD caching
means two extra disks for the cache (or what happens when the cache disk
fails?), and ZFS doesn't increase the number of SAS/SATA ports you have.
I'm not sure what happens if a read or write to the SLOG (the device
acting as a write cache) fails. Anyone?

That's why you can have the ZIL mirrored.

As for the reads, if a read from
L2ARC (the read cache on SSD in our case) fails, it's just ignored
(AFAIK) and data is read from the vdevs ("primary" storage).


How does it do the checksumming?  Read everything after it's been
written to verify?

Each time data is read from the disk, it is checksummed and the checksum
is compared with the value stored with the data. This way you know
whether data you just read is good or not, not just that it had been
written correctly.

Indeed, and this is one of the things that no RAID controller I'm aware of will do for you. A RAID card will read the data stripe and if that doesn't return an error from a disk, it will be assumed to be healthy. If you had a phantom write clobber one of the blocks in that stripe, your application will end up consuming corrupted data. ZFS prevents that from happening.

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