Messages in this thread | | | Date | Fri, 22 Apr 2011 09:57:32 -0700 | From | Sunil Mushran <> | Subject | Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags |
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On 04/22/2011 09:40 AM, Eric Blake wrote: > On 04/22/2011 10:28 AM, Sunil Mushran wrote: >> while(1) { >> read(block); >> if (block_all_zeroes) >> lseek(SEEK_DATA); >> } >> >> What's wrong with the above? If this is the case, even SEEK_HOLE >> is not needed but should be added as it is already in Solaris. > Because you don't know if the block is the same size as the minimum > hole, and because some systems require rather large holes (my Solaris > testing on a zfs system didn't have holes until 128k), that's a rather > large amount of reading just to prove that the block has all zeros to > know that it is even worth trying the lseek(SEEK_DATA). My gut feel is > that doing the lseek(SEEK_HOLE) up front coupled with seeking back to > the same position is more efficient than manually checking for a run of > zeros (less cache pollution, works with 4k read buffers without having > to know filesystem hole size).
Holes are an implementation detail.
cp can read whatever blocksize it chooses. If that block contains zero, it would signal cp that maybe it should SEEK_DATA and skip reading all those blocks. That's all. We are not trying to achieve perfection. We are just trying to reduce cpu waste.
If the fs supports SEEK_*, then great. If it does not, then it is no worse than before.
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