On 31.03.2016 1:15, Sage Weil wrote:
On Wed, 30 Mar 2016, Allen Samuels wrote:
[snip]
Time to talk about checksums.
First let's divide the world into checksums for data and checksums for
metadata -- and defer the discussion about checksums for metadata
(important, but one at a time...)
I believe it's a requirement that when checksums are enabled that 100%
of data reads must be validated against their corresponding checksum.
This leads you to conclude that you must store a checksum for each
independently readable piece of data.
I'm just worried about the size of metadata if we have 4k checksums but
have to read big extents anyway; cheaper to store a 4 byte checksum for
each compressed blob.
But do we really need to store checksums as metadata?
What's about pre(post)fixing 4K-4(?) blob with the checksum and store
this pair to the disk.
IMO we always need checksum values along with blob data thus let's store
and read them together.
This immediately eliminates the question about the granularity and
corresponding overhead...
Have I missed something?
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