06-16-2011
Nice ZFS summary:
ZFS - THe newest file system, explained in human terms
Compression just means:
- the data on disk is smaller
- the data is more random
- if the data is corrupted, a bigger part or the remainer of the file is likely lost (but this is not 1962 tape or 1969 disk, and there is backup, raid and mirroring!)
- Because it reads fewer pages of disk for more pages of RAM, and CPUs are hugely faster than disks, serial read write flow is faster. Write flow is a bit more CPU intensive, might not beat raw disk. However, ZFS buffers writes a lot, and who has only one CPU any more? LZJB should be much faster than gzip, but perhaps twice as large?
- Next point is a bit speculative, as I could not find an exact description of ZFS compression blocking: Random access requires reading compressed blocks on disk from the beginning, and, for writes, recompression of entire separate compressed block.
I wonder if they ever considered deferring compression of each file until the data goes quiescent?
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SYNC(2) BSD System Calls Manual SYNC(2)
NAME
sync -- synchronize disk block in-core status with that on disk
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
void
sync(void);
DESCRIPTION
The sync() function forces a write of dirty (modified) buffers in the block buffer cache out to disk. The kernel keeps this information in
core to reduce the number of disk I/O transfers required by the system. As information in the cache is lost after a system crash, kernel
thread ioflush ensures that dirty buffers are synced to disk eventually. By default, a dirty buffer is synced after 30 seconds, but some
filesystems exploit ioflush features to sync directory data and metadata faster (after 15 and 10 seconds, respectively).
The function fsync(2) may be used to synchronize individual file descriptor attributes.
CAUTIONS
Many modern disks contain write-back caches. In theory sync() flushes these. In practice there are many possible ways for this mechanism to
go astray. It is prudent (where possible) to allow a few seconds after syncing for everything to settle before e.g. turning off the power.
It may also be desirable to use dkctl(8) or scsictl(8) to disable the write-back cache entirely.
SEE ALSO
fsync(2), dkctl(8), scsictl(8), sync(8)
HISTORY
A sync() function call appeared in Version 6 AT&T UNIX.
Historically, sync() would schedule buffers for writing but not actually wait for the writes to finish. It was necessary to issue a second
or sometimes a third call to ensure that all buffers had in fact been written out. In NetBSD, sync() does not return until all buffers have
been written.
BSD
March 25, 2009 BSD