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Re: [linux-lvm] lvm partition on ramdisk



Final results FYI in this investigation: A recent Ubuntu release gets past the 1TB limit. I found binaries for dosfstools_2.11-2.1ubuntu3 at
 
http://packages.ubuntu.com/feisty/otherosfs/dosfstools
 
and unpacked them with ar -x. The source is in
 
https://launchpad.net/ubuntu/+source/dosfstools/2.11-2.1ubuntu3
 
With 4KB sectors, it defaults to 128KB clusters and reaches over 97% write speed on an 8TB volume. The ramdisk area needed is a little over 512MB, so if you use 768MB you get quite a bit of room for directories also on ramdisk, and with a little finesse you can even make the subdirectories lay themselves down on ramdisk. To be "Windows-legal" you could use 32KB clusters and a little over 2GB ramdisk (or a little over 1GB with one FAT). Linux is happy with the big clusters, and according to the design should actually be willing to go to 16TB.

Larry
 
On 5/13/08, Stuart D. Gathman <stuart bmsi com> wrote:
On Tue, 13 May 2008, Larry Dickson wrote:

> on the full, unpartitioned lv. Then it mounted, with the entire FAT on
> ramdisk, and wrote very fast because FAT32 (like DOS) lays down data in
> order from the start of a disk and does not skip around (I'd be interested
> if anyone knows any other file systems with that property).

The SysV filesystem put a fixed size inode table at the beginning of a
partition.  More modern filesystems from ext to reiser try to distribute
the meta-data to keep it closer to the data.  This is, of course, counter
productive when the beginning of a disk is significantly faster and seek-free
as in your setup.

Since ext3 inode placement is table driven (with the table in a magic inode),
there is probably a simple patch to mke2fs to create only one inode table at
the beginning of a drive.  In fact, I wonder if there is already an option...
looks like -g blocks_per_group might do the trick - assuming inodes are
at the beginning of a block group, rather than the middle.  If not,
a patch to mke2fs is needed to do what you want.

--
             Stuart D. Gathman <stuart bmsi com>
   Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.

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