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Date:	Tue, 21 Apr 2009 00:06:39 -0400
From:	Nick Dokos <nicholas.dokos@...com>
To:	linux-ext4@...r.kernel.org
cc:	nicholas.dokos@...com
Subject: 

Now that 64-bit e2fsck can run to completion on a (newly-minted, never
mounted) filesystem, here are some numbers. They must be taken with
a large grain of salt of course, given the unrealistict situation, but
they might be reasonable lower bounds of what one might expect.

First, the disks are 300GB SCSI 15K rpm - there are 28 disks per RAID
controller and they are striped into 2TiB volumes (that's a limitation
of the hardware). 16 of these volumes are striped together using LVM, to
make a 32TiB volume.

The machine is a four-slot quad core AMD box with 128GB of memory and
dual-port FC adapters.

The filesystem was created with default values for everything, except
that the resize_inode feature is turned off. I cleared caches before the
run.

# time e2fsck -n -f /dev/mapper/bigvg-bigvol
e2fsck 1.41.4-64bit (17-Apr-2009)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/mapper/bigvg-bigvol: 11/2050768896 files (0.0% non-contiguous), 128808243/8203075584 blocks

real	23m13.725s
user	23m8.172s
sys	0m4.323s

Most of the time (about 22 minutes) is in pass 5. I was taking snapshots
of

     /proc/<pid of e2fsck>/statm

every 10 seconds during the run[1]. It starts out like this:

27798 3293 217 42 0 3983 0
609328 585760 263 42 0 585506 0
752059 728469 272 42 0 728237 0
752059 728469 272 42 0 728237 0
752059 728469 272 42 0 728237 0
752059 728469 272 42 0 728237 0
752059 728469 272 42 0 728237 0
752059 728469 272 42 0 728237 0
752059 728469 272 42 0 728237 0
717255 693666 273 42 0 693433 0
717255 693666 273 42 0 693433 0
717255 693666 273 42 0 693433 0
....

and stays at that level for most of the run (the drop occurs a short
time after pass 5 starts). Here is what it looks like at the end:

....
717255 693666 273 42 0 693433 0
717255 693666 273 42 0 693433 0
717255 693666 273 42 0 693433 0
717499 693910 273 42 0 693677 0
717499 693910 273 42 0 693677 0
717499 693910 273 42 0 693677 0

So in this very simple case, memory required tops out at about 3 GB for the
32Tib filesystem, or 0.4 bytes per block.

Nick

[1] The numbers are numbers of pages. The format is described in
Documentation/filesystems/proc.txt:

Table 1-2: Contents of the statm files (as of 2.6.8-rc3)
..............................................................................
 Field    Content
 size     total program size (pages)		(same as VmSize in status)
 resident size of memory portions (pages)	(same as VmRSS in status)
 shared   number of pages that are shared	(i.e. backed by a file)
 trs      number of pages that are 'code'	(not including libs; broken,
							includes data segment)
 lrs      number of pages of library		(always 0 on 2.6)
 drs      number of pages of data/stack		(including libs; broken,
							includes library text)
 dt       number of dirty pages			(always 0 on 2.6)
..............................................................................
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