Stephen Hahn wrote:
* Moinak Ghosh <[EMAIL PROTECTED]> [2007-06-25 07:36]:
ZFS is unsuitable for a LiveCD. In fact for a LiveCD all that is
required is a minimal, ramdisk based R/W filesystem and a CDROM
filesystem. For that matter even UFS is a bit heavyweight for a
ramdisk based filesystem.
Could you give more technical detail on ZFS's lack of suitability as a
LiveCD filesystem?
There are several drawbacks not the least of which is the amount of
resources
demanded by ZFS. In a LiveCD env we are already blocking up some RAM
because of a ramdisk and typically no physical swap, so the
filesystem eating
up additional RAM for extra caching etc. is bad idea. In addition 256
- 512 MB
RAM is a quite common config.
In fact for a ramdisk based fs we just need some minimal thing that
provides
all the standard semantics. ZFS has too many features to be useful
for a ramdisk!
Something much more smaller than UFS would in fact be great.
End-to-end data integrity, checksumming, advanced caching,
snapshots, pooled
storage, ZIL etc. simply add unnecessary overhead for a transient
ramdisk - these
are all killer things to have for your harddisk[s].
Same is true for the other component that is the CDROM. Lofi compression
with the enhanced HSFS sitting on top was observed to give better
results than a
compressed zpool. One of the big things that HSFS allows is file data
re-ordering.
We can profile the bootup (using DTrace) and compute improved
placement of
file data on the CDROM. Makes a huge difference. Once can't do this
with ZFS.
In fact it is possible to do compressed segment level profiling and
reorder due to
the index in lofi compression. Though I have not yet done that for
BeleniX.
The lofi compression technique also allows efficient, explicit
preloading (via pagein)
without having to decompress every page.
The I/O Scheduler in ZFS also does not appear to be optimal for
spiral track
CD/DVD media. It always does an avl_first of the deadline tree to get
the next
I/O request to service and coalesces backward and forward. This can
cause a lot
of CDROM head seeking as it is sequential seek in nature. C-SCAN or
it's variant
called C-LOOK scheduling appears to better suited for CD/DVD. It is
mentioned
in a couple of papers:
http://www-cse.ucsd.edu/groups/multimedia/papers/srjrs.ps
http://portal.acm.org/citation.cfm?id=846220.847543&coll=&dl=&CFID=15151515&CFTOKEN=6184618
Regards,
Moinak,
- Stephen
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