Hi, Nathan and Sebastian,
I have drafted a formula for the bmap->LBA mapping for a simple stripe
group(RAID0) on IRIX/CXFS. Maybe it is some roughness,
If there is any mistake, correct me:
For example: if a file is on [200, 1000]( shown from xfs_bmap). And the
stripe unit is 128kB.
stripe width is 3(totally 3 disks), xvm can told us the user data area
begin after 4096 block.(This is changable by admin)
stripe_unit_size = 128KB
stripe_width = 3
sector_size = 512Btyes
chuck_blocks = stripe_unit_size/sector_size = 128KB/512B = 256 blocks.
stripe_line_blocks = chuck_block * stripe_width = 256 * 3 = 768 blocks
data_offset_to_disk_block0 = 4096 blocks.
file_sector_offset_in_stripe_line = BMAP % stripe_line_blocks = 200 %
768 = 200
disk_id = file_sector_offset_in_stripe_line / chunk_blocks = 200 / 256 = 0
stripe_high = BMAP / strip_line_blocks = 200 / 768 = 0
File_head_sector = stripe_high * chuck_blocks +
data_offset_to_disk_block0 + file_sector_offset_in_stripe_line
= 0*256 + 4096 + 200 = 4296
The LBA for the file's first block IS the above line: 4296.
To ensure it, just dd back the raw data from the disk at the offset 4296
is ok.
However, I am not sure the result if there is a plex layer is inserted
or on the realtime volume...
Could you or other interested persons refine the calculation for some
variation or is there any outstanding issue on it?
Thanks,
Michael
Sebastian Brings wrote:
For Irix I am aware of two ways. One is to do the math, using xvm show
-verbose phys/* and so on to get the sizes of labels and volume header
and figure out where the first data block starts.
The other way is to use a program which does direct IO read on an
existing file and run it under control of "par -k". This produces plenty
of output, but holds information like "read from device X starting at
offset Y number of bytes Z. You then can match it with your xfs_bmap -v
list.
Cheers
Sebastian
-----Original Message-----
From: xfs-bounce@xxxxxxxxxxx [mailto:xfs-bounce@xxxxxxxxxxx] On Behalf
Of
Nathan Scott
Sent: Freitag, 14. Juli 2006 07:31
To: Michael Li (gmail)
Cc: xfs@xxxxxxxxxxx
Subject: Re: XFS bmap to disk lba question.
On Fri, Jul 14, 2006 at 01:58:04AM +0800, Michael Li (gmail) wrote:
We know that the command xfs_bmap can show us the file's extent
range,
for example [20 - 100], but how can we know the real physical
secoter ID
(or
named as LBA) of file's first block(512Bytes) on a raw disk? I've
read
XVM admin for IRIX, it show us less clue for this mapping.
For a single disk, its relatively simple. bmap gives you the starting
offset for each extent in basic blocks (512 bytes) always, which maps
directly to sectors on the logical device.
You need to consider any partition table or volume manager header at
the start of a physical device, and factor that into the calculation,
they are not any one fixed size (I don't know how large an XVM label
would be for example).
We can get this mapping method on linux, as linux/xfs is open
sourced,
but IRIX is not, we don't know how to do it on IRIX, although the
filesystem is the in the same name XFS.
The xfs_bmap command is basically the same between IRIX and Linux.
Furthermore, is it the same way to mapping the bmap/LBA in a striped
volume. ...
unit is 128. each stripe unit has 32 512B-blocks. How can we map the
first block in the file to one of disk's X sector?
Well, it gets more complex now of course, bmap gives you one number
(for the single logical address space presented by the raid or volume
manager), and its an exercise for the reader to figure out which
actual disk that corresponds to based on the raid geometry. Theres
no tools for doing this that I know of, you just have to sit down and
do the math... (well, thats how I've done it in the past anyway)... a
bit of a pain, I know.
BTW: If we should not talk about IRIX here, I will stop posting such
a
topic here. Sorry for the noise.
Heh, yes, its more usual to speak to SGI customer support folks for
this sort of topic.
cheers.
--
Nathan
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