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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
