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

[Kuba <kuba.0000@xxxxx>]

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? 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. Take a look here for example, slides 12-16:
http://wiki.illumos.org/download/attachments/1146951/zfs_last.pdf

It's a little bit outdated and Solaris-centric, but might give you some 
overview.
Kuba

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