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blocks of a file as contiguously as possible. Since the filesystem does not know how large a file will grow when it is first being written, it initially places the file in a set of blocks in which it currently fits. As it grows, it is relocated to areas with larger contiguous blocks. In this way it saves its large contiguous sets of blocks for the files that need them and thus avoids unnecessaily fragmenting its disk space. We used to skip reallocating the blocks of a file into a contiguous sequence if the underlying flash device requested BIO_DELETE notifications, because devices that benefit from BIO_DELETE also benefit from not moving the data. However, in the algorithm described above that reallocates the blocks, the destination for the data is usually moved before the data is written to the initially allocated location. So we rarely suffer the penalty of extra writes. With the addition of the consolodation of contiguous blocks into single BIO_DELETE operations, having fewer but larger contiguous blocks reduces the number of (slow and expensive) BIO_DELETE operations. So when doing BIO_DELETE consolodation, we do block reallocation. Reviewed by: kib Tested by: Peter Holm Sponsored by: Netflix
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