110 lines
5.3 KiB
Plaintext
110 lines
5.3 KiB
Plaintext
# $NetBSD: TODO,v 1.10 2005/12/11 12:25:26 christos Exp $
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- Lock audit. Need to check locking for multiprocessor case in particular.
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- Get rid of lfs_segclean(); the kernel should clean a dirty segment IFF it
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has passed two checkpoints containing zero live bytes.
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- Now that our cache is basically all of physical memory, we need to make
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sure that segwrite is not starving other important things. Need a way
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to prioritize which blocks are most important to write, and write only
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those, saving the rest for later. Does this change our notion of what
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a checkpoint is?
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- Investigate alternate inode locking strategy: Inode locks are useful
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for locking against simultaneous changes to inode size (balloc,
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truncate, write) but because the assignment of disk blocks is also
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covered by the segment lock, we don't really need to pay attention to
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the inode lock when writing a segment, right? If this is true, the
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locking problem in lfs_{bmapv,markv} goes away and lfs_reserve can go,
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too.
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- Get rid of DEV_BSIZE, pay attention to the media block size at mount time.
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- More fs ops need to call lfs_imtime. Which ones? (Blackwell et al., 1995)
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- lfs_vunref_head exists so that vnodes loaded solely for cleaning can
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be put back on the *head* of the vnode free list. Make sure we
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actually do this, since we now take IN_CLEANING off during segment write.
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- The cleaner could be enhanced to be controlled from other processes,
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and possibly perform additional tasks:
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- Backups. At a minimum, turn the cleaner off and on to allow
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effective live backups. More aggressively, the cleaner itself could
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be the backup agent, and dump_lfs would merely be a controller.
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- Cleaning time policies. Be able to tweak the cleaner's thresholds
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to allow more thorough cleaning during policy-determined idle
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periods (regardless of actual idleness) or put off until later
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during short, intensive write periods.
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- File coalescing and placement. During periods we expect to be idle,
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coalesce fragmented files into one place on disk for better read
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performance. Ideally, move files that have not been accessed in a
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while to the extremes of the disk, thereby shortening seek times for
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files that are accessed more frequently (though how the cleaner
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should communicate "please put this near the beginning or end of the
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disk" to the kernel is a very good question; flags to lfs_markv?).
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- Versioning. When it cleans a segment it could write data for files
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that were less than n versions old to tape or elsewhere. Perhaps it
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could even write them back onto the disk, although that requires
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more thought (and kernel mods).
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- Move lfs_countlocked() into vfs_bio.c, to replace count_locked_queue;
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perhaps keep the name, replace the function. Could it count referenced
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vnodes as well, if it was in vfs_subr.c instead?
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- Why not delete the lfs_bmapv call, just mark everything dirty that
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isn't deleted/truncated? Get some numbers about what percentage of
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the stuff that the cleaner thinks might be live is live. If it's
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high, get rid of lfs_bmapv.
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- There is a nasty problem in that it may take *more* room to write the
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data to clean a segment than is returned by the new segment because of
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indirect blocks in segment 2 being dirtied by the data being copied
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into the log from segment 1. The suggested solution at this point is
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to detect it when we have no space left on the filesystem, write the
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extra data into the last segment (leaving no clean ones), make it a
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checkpoint and shut down the file system for fixing by a utility
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reading the raw partition. Argument is that this should never happen
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and is practically impossible to fix since the cleaner would have to
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theoretically build a model of the entire filesystem in memory to
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detect the condition occurring. A file coalescing cleaner will help
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avoid the problem, and one that reads/writes from the raw disk could
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fix it.
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- Need to keep vnode v_numoutput up to date for pending writes?
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- If delete a file that's being executed, the version number isn't
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updated, and fsck_lfs has to figure this out; case is the same as if
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have an inode that no directory references, so the file should be
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reattached into lost+found.
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- Currently there's no notion of write error checking.
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+ Failed data/inode writes should be rescheduled (kernel level bad blocking).
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+ Failed superblock writes should cause selection of new superblock
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for checkpointing.
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- Future fantasies:
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- unrm, versioning
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- transactions
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- extended cleaner policies (hot/cold data, data placement)
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- Problem with the concept of multiple buffer headers referencing the segment:
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Positives:
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Don't lock down 1 segment per file system of physical memory.
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Don't copy from buffers to segment memory.
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Don't tie down the bus to transfer 1M.
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Works on controllers supporting less than large transfers.
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Disk can start writing immediately instead of waiting 1/2 rotation
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and the full transfer.
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Negatives:
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Have to do segment write then segment summary write, since the latter
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is what verifies that the segment is okay. (Is there another way
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to do this?)
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- The algorithm for selecting the disk addresses of the super-blocks
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has to be available to the user program which checks the file system.
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