gvinum raid5 vs. ZFS raidz

[Scott Bennett]

Trond Endrest?l <Trond.Endrestol at fagskolen.gjovik.no> wrote:
> On Thu, 7 Aug 2014 03:31-0500, Scott Bennett wrote:
> > Arthur Chance <freebsd at qeng-ho.org> wrote:
> > > On 06/08/2014 06:56, Scott Bennett wrote:
> > > > Arthur Chance <freebsd at qeng-ho.org> wrote:
> > > >> 
> > > >> [stuff deleted --SB]
> > > >       I wonder if what varies is the amount of space taken up by the
> > > > checksums.  If there's a checksum for each block, then the block size
> > > > would change the fraction of the space lost to checksums, and the parity
> > > > for the checksums would thus also change.  Enough to matter?  Maybe.
> > >
> > > I'm not a file system guru, but my (high level) understanding is as 
> > > follows. Corrections from anyone more knowledgeable welcome.
> > >
> > > 1. UFS and ZFS both use tree structures to represent files, with the 
> > > data stored at the leaves and bookkeeping stored in the higher nodes. 
> > > Therefore the overhead scales as the log of the data size, which is a 
> > > negligible fraction for any sufficiently large amount of data.
> > >
> > > 2. UFS doesn't have data checksums, it relies purely on the hardware 
> > > checksums. (This is the area I'm least certain of.)
> > 
> >      What hardware checksums are there?  I wasn't aware that this sort of
> > hardware kept any.
>
> To quote http://en.wikipedia.org/wiki/Disk_sector:
>
> In disk drives, each physical sector is made up of three basic parts, 
> the sector header, the data area and the error-correcting code (ECC).

     That's interesting, and I know it was true in the days of minicomputers.
However, it appears to be out of date, based upon 1) the observed fact that
corrupted data *do* get recorded onto today's PC-style disk drives with no
indication that an error has occurred, no parity bits are present in the
processor chips, memory cards, motherboards, PATA/SATA/SCSI/etc. controllers,
nor 2) the disk drives themselves, as confirmed by the technical support guy
I spoke with about it at Seagate/Samsung recently.  That guy said that there
is *no parity-checking* of data written to/read from the disks and that some
silent errors are now considered to be "normal" on disks whose capacities
exceed 1 TB.
>
> [remainder deleted  --SB]


                                  Scott Bennett, Comm. ASMELG, CFIAG
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