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is to first write the deleted directory entry to disk, second write the zero'ed inode to disk, and finally to release the freed blocks and the inode back to the cylinder-group map. As this ordering requires two disk writes to occur which are normally spaced about 30 seconds apart (except when memory is under duress), it takes about a minute from the time that a file is deleted until its inode and data blocks show up in the cylinder-group map for reallocation. If a file has had only a brief lifetime (less than 30 seconds from creation to deletion), neither its inode nor its directory entry may have been written to disk. If its directory entry has not been written to disk, then we need not wait for that directory block to be written as the on-disk directory block does not reference the inode. Similarly, if the allocated inode has never been written to disk, we do not have to wait for it to be written back either as its on-disk representation is still zero'ed out. Thus, in the case of a short lived file, we can simply release the blocks and inode to the cylinder-group map immediately. As the inode and its blocks are released immediately, they are immediately available for other uses. If they are not released for a minute, then other inodes and blocks must be allocated for short lived files, cluttering up the vnode and buffer caches. The previous code was a bit too aggressive in trying to release the blocks and inode back to the cylinder-group map resulting in their being made available when in fact the inode on disk had not yet been zero'ed. This patch takes a more conservative approach to doing the release which avoids doing the release prematurely. |
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