[linux-lvm] lvm partition on ramdisk

[Larry Dickson]

Rereading your question more precisely, the performances were (write speeds,
as percentage of long sequential writes to raw device, 64GB timed, with
pre-filling of buffers in all tests):

97%    ramdisk-assisted FAT32
79%    ext2 (fastest non-ramdisk-assisted FS)
59%    FAT32
58%    xfs

So, in respect of performance loss, the ramdisk results in at least a
tenfold improvement. In the test, not only the FAT but the directory was on
ramdisk - and this is realistic.

Larry

On 5/19/08, Larry Dickson <ldickson at cuttedge.com> wrote:
>
> The purpose was to move seeking operations onto the ramdisk (ultimately
> NVRAM or battery-backed), and use a file system that allocates disk sectors
> in sequential order for scientific (no-delete) data streaming writes. It
> allows near full speed writing to the main (big RAID) storage, and yet a
> standard file system structure appears on the recorded data at little cost.
> Log-structured file systems serve a similar purpose, but the structure of
> the data and metadata is quite complex, and an additional step of "capping"
> is needed if random-access reads are to be reasonably fast.
>
> Larry
>
>  On 5/19/08, Gerrard Geldenhuis <Gerrard.Geldenhuis at datacash.com> wrote:
>>
>>  Thanks for the update. I have not followed the thread closely… did you
>> see any significant measured peformance increase in using lvm on a ramdisk?
>>
>>
>>
>> Regards
>>
>>
>>   ------------------------------
>>
>> *From:* linux-lvm-bounces at redhat.com [mailto:linux-lvm-bounces at redhat.com]
>> *On Behalf Of *Larry Dickson
>> *Sent:* 19 May 2008 15:37
>> *To:* LVM general discussion and development
>> *Subject:* 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 at 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 at 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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>> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>>
>>
>>
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>>
>
>
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