On Fri, Feb 6, 2015 at 4:30 PM, Michael Paquier wrote: > On Fri, Feb 6, 2015 at 3:03 PM, Fujii Masao wrote: >> Do we always need extra two bytes for compressed backup block? >> ISTM that extra bytes are not necessary when the hole length is zero. >> In this case the length of the original backup block (i.e., uncompressed) >> must be BLCKSZ, so we don't need to save the original size in >> the extra bytes. > > Yes, we would need a additional bit to identify that. We could steal > it from length in XLogRecordBlockImageHeader. > >> Furthermore, when fpw compression is disabled and the hole length >> is zero, we seem to be able to save one byte from the header of >> backup block. Currently we use 4 bytes for the header, 2 bytes for >> the length of backup block, 15 bits for the hole offset and 1 bit for >> the flag indicating whether block is compressed or not. But in that case, >> the length of backup block doesn't need to be stored because it must >> be BLCKSZ. Shouldn't we optimize the header in this way? Thought? > > If we do it, that's something to tackle even before this patch on > HEAD, because you could use the 16th bit of the first 2 bytes of > XLogRecordBlockImageHeader to do necessary sanity checks, to actually > not reduce record by 1 byte, but 2 bytes as hole-related data is not > necessary. I imagine that a patch optimizing that wouldn't be that > hard to write as well.
Actually, as Heikki pointed me out... A block image is 8k and pages without holes are rare, so it may be not worth sacrificing code simplicity for record reduction at the order of 0.1% or smth like that, and the current patch is light because it keeps things simple. -- Michael -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers