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bigallocbigallocPosted Nov 30, 2011 20:02 UTC (Wed) by iabervon (subscriber, #722)In reply to: bigalloc by tialaramex Parent article: Improving ext4: bigalloc, inline data, and metadata checksums
My impression is that bigalloc doesn't affect the unit of caching. Rather, it affects the unit of disk block allocation, meaning that pages 0-1023 are adjacent on your SSD and the filesystem metadata specifies that that 4M is in use and the inode has a single disk location to find it, but the pages are still accessed independently.
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bigalloc Posted Nov 30, 2011 21:16 UTC (Wed) by walex (subscriber, #69836) [Link] Note that 'ext4' supports extents, so files can get allocated with very large contiguous extents already, for example for a 70MB file: # du -sm /usr/share/icons/oxygen/icon-theme.cache 69 /usr/share/icons/oxygen/icon-theme.cache # filefrag /usr/share/icons/oxygen/icon-theme.cache /usr/share/icons/oxygen/icon-theme.cache: 1 extent found # df -T /usr/share/icons/oxygen/icon-theme.cache Filesystem Type 1M-blocks Used Available Use% Mounted on /dev/sda3 ext4 25383 12558 11545 53% / But so far the free space has been tracked in block-sized units, and the new thing seems to change the amount of free space accounted for by each bit in the free space bitmap. Which means that as surmised the granularity of allocation has changed (for example minimum extent size).
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