On 02/16/2011 04:44 PM, Roman Mamedov wrote:
On Thu, 17 Feb 2011 08:24:12 +1100
NeilBrown<neilb@xxxxxxx> wrote:
"read/write/compare checksum" is not a lot of words so I may well not be
understanding exactly what you mean, but I guess you are suggesting that we
could store (say) a 64bit hash of each 4K block somewhere.
e.g. Use 513 4K blocks to store 512 4K blocks of data with checksums.
When reading a block, read the checksum too and report an error if they
don't match. When writing the block, calculate and write the checksum too.
This is already done by the disk drive - I'm not sure what you hope to gain
by doing it in the RAID layer as well.
Consider RAID1/RAID10/RAID5/RAID6, where one or more members are returning bad
data for some reason (e.g. are failing or have written garbage to disk during
a sudden power loss). Having per-block checksums would allow to determine
which members have correct data and which do not, and would help the RAID
layer recover from that situation in the smartest way possible (with absolutely
no loss or corruption of the user data).
I wasn't specifically thinking about bad data from a power loss, but the
more general case of something in the pathway causing bad bits to have
been committed or read back from the storage. I am after being able to
detect bad reads (silent corruption) and bad writes (by flushing then
reading recently written blocks to compare).
Suppose, for example, we have a RAID1, and we read block N. As a sanity
check on the data, we can compare the data read from one device to
another. This doesn't tell us if the data is correct, just whether or
not the same data was returned. So the RAID layer (nor the disks
themselves) would return an error in the event of this data not matching
a checksum. So if we computed a simple checksum compared it to a stored
checksum, we could likely detect corruption on read.
Similar to this would be computing/comparing the RAIDn (n>1) checksum on
every read. It would cost somewhat more processing power, but I believe
that in most cases, the disk performance would be the rate limiting process.
It might make more sense to push some of this up to the file system
layers (ala btrfs), but I am thinking that it would be nice to have some
elements of this functionality in the RAID layers, that the upper level
file systems can use as a service.
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
