:The "md" driver I committed today is mostly a proof-of-concept thing
:which came out of a chat with Peter Wemm.
:
:It acts like a disk in all aspects, although it will be hard to boot
:from it :-)
:
:It will do very simple compression, in that a sector (512bytes)
:which is filled with the same character throughout, will not be
:allocated as a full sector, instead just the byte value is saved.
:
:In practice this means that making a 10M filesystem doesn't take
:10M of ram, until you fill data into it. The driver will also free
:the sectors after you delete a file from the filesystem (ufs only).
:
:# disklabel -r -w md0 auto
:# newfs md0c
:# vmstat -m | grep " MD s"
: MD sectors 635 318K 318K 21136K 635 0 0 512
:# mount /dev/md0c /mnt
:# cp /kernel /mnt
:# vmstat -m | grep " MD s"
: MD sectors 4178 2089K 2089K 21136K 4178 0 0 512
:# rm /mnt/kernel
:# vmstat -m | grep " MD s"
: MD sectors 636 318K 2120K 21136K 4240 0 0 512
:# umount /mnt
:# dd if=/dev/zero of=/dev/rmd0
:# vmstat -m | grep " MD s"
: MD sectors 0 0K 2120K 21136K 4240 0 0 512
:#
That's very interesting. You can do the same thing with the VN
device, using swap-backing, except it does not do any compression.
test3:/root# vnconfig -s labels -c -S 32g vn0
test3:/root# pstat -s
Device 1K-blocks Used Avail Capacity Type
/dev/od0b 1048448 0 1048448 0% Interleaved
/dev/da1b 1048448 0 1048448 0% Interleaved
/dev/da2b 1048448 0 1048448 0% Interleaved
Total 3145344 0 3145344 0%
test3:/root# disklabel -r -w vn0 auto
test3:/root# newfs /dev/rvn0c
Warning: Block size restricts cylinders per group to 8.
/dev/rvn0c: 67108864 sectors in 2048 cylinders of 1 tracks, 32768 sectors
32768.0MB in 256 cyl groups (8 c/g, 128.00MB/g, 8128 i/g)
super-block backups (for fsck -b #) at:
32, 262176, 524320, 786464, 1048608, 1310752, 1572896, 1835040, 2097184,
2359328, 2621472, 2883616, 3145760, 3407904, 3670048, 3932192, 4194336,
4456480, 4718624, 4980768, 5242912, 5505056, 5767200, 6029344, 6291488,
...
62128160, 62390304, 62652448, 62914592, 63176736, 63438880, 63701024,
63963168, 64225312, 64487456, 64749600, 65011744, 65273888, 65536032,
65798176, 66060320, 66322464, 66584608, 66846752
test3:/root# pstat -s
Device 1K-blocks Used Avail Capacity Type
/dev/od0b 1048448 88092 960356 8% Interleaved
/dev/da1b 1048448 88064 960384 8% Interleaved
/dev/da2b 1048448 88064 960384 8% Interleaved
Total 3145344 264220 2881124 8%
test3:/root# mount /dev/vn0c /mnt
test3:/root# cp /kernel /mnt
test3:/root#
test3:/root# pstat -s
Device 1K-blocks Used Avail Capacity Type
/dev/od0b 1048448 88764 959684 8% Interleaved
/dev/da1b 1048448 88704 959744 8% Interleaved
/dev/da2b 1048448 88704 959744 8% Interleaved
Total 3145344 266172 2879172 8%
test3:/root# rm /mnt/kernel
test3:/root# pstat -s
Device 1K-blocks Used Avail Capacity Type
/dev/od0b 1048448 88092 960356 8% Interleaved
/dev/da1b 1048448 88064 960384 8% Interleaved
/dev/da2b 1048448 88064 960384 8% Interleaved
Total 3145344 264220 2881124 8%
test3:/root# umount /mnt
test3:/root# vnconfig -u vn0
test3:/root# pstat -s
Device 1K-blocks Used Avail Capacity Type
/dev/od0b 1048448 0 1048448 0% Interleaved
/dev/da1b 1048448 0 1048448 0% Interleaved
/dev/da2b 1048448 0 1048448 0% Interleaved
Total 3145344 0 3145344 0%
However, for general use it is recommended that one use the
'reserve' flag for swap-backed VN filesystems to pre-reserve all
necessary swap in order to maintain performance. And enabling
softupdates on it once you've newfs'd is also a good idea.
Just for kicks:
test3:/root# vnconfig -s labels -c -S 4t vn0
test3:/root# disklabel -r -w vn0 auto
test3:/root# disklabel vn0
...
bytes/sector: 4096
8 partitions:
# size offset fstype [fsize bsize bps/cpg]
c: 1073741824 0 unused 0 0 # (Cyl. 0 - 4194303)
test3:/root# pstat -s
Device 1K-blocks Used Avail Capacity Type
Total 3145344 8 3145336 0%
test3:/root# newfs /dev/rvn0c
preposterous size 0 <---------------------------
I find that totally amusing. I think I'm going to fix newfs, it really
should be able to handle a 4 terrabyte filesystem.
:--
:Poul-Henning Kamp FreeBSD coreteam member
:[EMAIL PROTECTED] "Real hackers run -current on their laptop."
-Matt
Matthew Dillon
<[EMAIL PROTECTED]>
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