[gpfsug-discuss] Blocksize
Yuri L Volobuev
volobuev at us.ibm.com
Mon Sep 26 19:18:15 BST 2016
It's important to understand the differences between different metadata
types, in particular where it comes to space allocation.
System metadata files (inode file, inode and block allocation maps, ACL
file, fileset metadata file, EA file in older versions) are allocated at
well-defined moments (file system format, new storage pool creation in the
case of block allocation map, etc), and those contain multiple records
packed into a single block. From the block allocator point of view, the
individual metadata record size is invisible, only larger blocks get
actually allocated, and space usage efficiency generally isn't an issue.
For user metadata (indirect blocks, directory blocks, EA overflow blocks)
the situation is different. Those get allocated as the need arises,
generally one at a time. So the size of an individual metadata structure
matters, a lot. The smallest unit of allocation in GPFS is a subblock
(1/32nd of a block). If an IB or a directory block is smaller than a
subblock, the unused space in the subblock is wasted. So if one chooses to
use, say, 16 MiB block size for metadata, the smallest unit of space that
can be allocated is 512 KiB. If one chooses 1 MiB block size, the smallest
allocation unit is 32 KiB. IBs are generally 16 KiB or 32 KiB in size (32
KiB with any reasonable data block size); directory blocks used to be
limited to 32 KiB, but in the current code can be as large as 256 KiB. As
one can observe, using 16 MiB metadata block size would lead to a
considerable amount of wasted space for IBs and large directories (small
directories can live in inodes). On the other hand, with 1 MiB block size,
there'll be no wasted metadata space. Does any of this actually make a
practical difference? That depends on the file system composition, namely
the number of IBs (which is a function of the number of large files) and
larger directories. Calculating this scientifically can be pretty
involved, and really should be the job of a tool that ought to exist, but
doesn't (yet). A more practical approach is doing a ballpark estimate
using local file counts and typical fractions of large files and
directories, using statistics available from published papers.
The performance implications of a given metadata block size choice is a
subject of nearly infinite depth, and this question ultimately can only be
answered by doing experiments with a specific workload on specific
hardware. The metadata space utilization efficiency is something that can
be answered conclusively though.
yuri
From: "Buterbaugh, Kevin L" <Kevin.Buterbaugh at Vanderbilt.Edu>
To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>,
Date: 09/24/2016 07:19 AM
Subject: Re: [gpfsug-discuss] Blocksize
Sent by: gpfsug-discuss-bounces at spectrumscale.org
Hi Sven,
I am confused by your statement that the metadata block size should be 1 MB
and am very interested in learning the rationale behind this as I am
currently looking at all aspects of our current GPFS configuration and the
possibility of making major changes.
If you have a filesystem with only metadataOnly disks in the system pool
and the default size of an inode is 4K (which we would do, since we have
recently discovered that even on our scratch filesystem we have a bazillion
files that are 4K or smaller and could therefore have their data stored in
the inode, right?), then why would you set the metadata block size to
anything larger than 128K when a sub-block is 1/32nd of a block? I.e.,
with a 1 MB block size for metadata wouldn’t you be wasting a massive
amount of space?
What am I missing / confused about there?
Oh, and here’s a related question … let’s just say I have the above
configuration … my system pool is metadata only and is on SSD’s. Then I
have two other dataOnly pools that are spinning disk. One is for “regular”
access and the other is the “capacity” pool … i.e. a pool of slower storage
where we move files with large access times. I have a policy that says
something like “move all files with an access time > 6 months to the
capacity pool.” Of those bazillion files less than 4K in size that are
fitting in the inode currently, probably half a bazillion (<grin>) of them
would be subject to that rule. Will they get moved to the spinning disk
capacity pool or will they stay in the inode??
Thanks! This is a very timely and interesting discussion for me as well...
Kevin
On Sep 23, 2016, at 4:35 PM, Sven Oehme <oehmes at us.ibm.com> wrote:
your metadata block size these days should be 1 MB and there are only
very few workloads for which you should run with a filesystem
blocksize below 1 MB. so if you don't know exactly what to pick, 1 MB
is a good starting point.
the general rule still applies that your filesystem blocksize
(metadata or data pool) should match your raid controller (or GNR
vdisk) stripe size of the particular pool.
so if you use a 128k strip size(defaut in many midrange storage
controllers) in a 8+2p raid array, your stripe or track size is 1 MB
and therefore the blocksize of this pool should be 1 MB. i see many
customers in the field using 1MB or even smaller blocksize on RAID
stripes of 2 MB or above and your performance will be significant
impacted by that.
Sven
------------------------------------------
Sven Oehme
Scalable Storage Research
email: oehmes at us.ibm.com
Phone: +1 (408) 824-8904
IBM Almaden Research Lab
------------------------------------------
<graycol.gif>Stephen Ulmer ---09/23/2016 12:16:34 PM---Not to be too
pedantic, but I believe the the subblock size is 1/32 of the block
size (which strengt
From: Stephen Ulmer <ulmer at ulmer.org>
To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date: 09/23/2016 12:16 PM
Subject: Re: [gpfsug-discuss] Blocksize
Sent by: gpfsug-discuss-bounces at spectrumscale.org
Not to be too pedantic, but I believe the the subblock size is 1/32
of the block size (which strengthens Luis’s arguments below).
I thought the the original question was NOT about inode size, but
about metadata block size. You can specify that the system pool have
a different block size from the rest of the filesystem, providing
that it ONLY holds metadata (—metadata-block-size option to mmcrfs).
So with 4K inodes (which should be used for all new filesystems
without some counter-indication), I would think that we’d want to use
a metadata block size of 4K*32=128K. This is independent of the
regular block size, which you can calculate based on the workload if
you’re lucky.
There could be a great reason NOT to use 128K metadata block size,
but I don’t know what it is. I’d be happy to be corrected about this
if it’s out of whack.
--
Stephen
On Sep 22, 2016, at 3:37 PM, Luis Bolinches <
luis.bolinches at fi.ibm.com> wrote:
Hi
My 2 cents.
Leave at least 4K inodes, then you get massive
improvement on small files (less 3.5K minus whatever you
use on xattr)
About blocksize for data, unless you have actual data
that suggest that you will actually benefit from smaller
than 1MB block, leave there. GPFS uses sublocks where
1/16th of the BS can be allocated to different files, so
the "waste" is much less than you think on 1MB and you
get the throughput and less structures of much more data
blocks.
No warranty at all but I try to do this when the BS talk
comes in: (might need some clean up it could not be last
note but you get the idea)
POSIX
find . -type f -name '*' -exec ls -l {} \; >
find_ls_files.out
GPFS
cd /usr/lpp/mmfs/samples/ilm
gcc mmfindUtil_processOutputFile.c -o
mmfindUtil_processOutputFile
./mmfind /gpfs/shared -ls -type f > find_ls_files.out
CONVERT to CSV
POSIX
cat find_ls_files.out | awk '{print $5","}' >
find_ls_files.out.csv
GPFS
cat find_ls_files.out | awk '{print $7","}' >
find_ls_files.out.csv
LOAD in octave
FILESIZE = int32 (dlmread ("find_ls_files.out.csv",
","));
Clean the second column (OPTIONAL as the next clean up
will do the same)
FILESIZE(:,[2]) = [];
If we are on 4K aligment we need to clean the files that
go to inodes (WELL not exactly ... extended attributes!
so maybe use a lower number!)
FILESIZE(FILESIZE<=3584) =[];
If we are not we need to clean the 0 size files
FILESIZE(FILESIZE==0) =[];
Median
FILESIZEMEDIAN = int32 (median (FILESIZE))
Mean
FILESIZEMEAN = int32 (mean (FILESIZE))
Variance
int32 (var (FILESIZE))
iqr interquartile range, i.e., the difference between the
upper and lower quartile, of the input data.
int32 (iqr (FILESIZE))
Standard deviation
For some FS with lots of files you might need a rather
powerful machine to run the calculations on octave, I
never hit anything could not manage on a 64GB RAM Power
box. Most of the times it is enough with my laptop.
--
Ystävällisin terveisin / Kind regards / Saludos
cordiales / Salutations
Luis Bolinches
Lab Services
http://www-03.ibm.com/systems/services/labservices/
IBM Laajalahdentie 23 (main Entrance) Helsinki, 00330
Finland
Phone: +358 503112585
"If you continually give you will continually have."
Anonymous
----- Original message -----
From: Stef Coene <stef.coene at docum.org>
Sent by: gpfsug-discuss-bounces at spectrumscale.org
To: gpfsug main discussion list <
gpfsug-discuss at spectrumscale.org>
Cc:
Subject: Re: [gpfsug-discuss] Blocksize
Date: Thu, Sep 22, 2016 10:30 PM
On 09/22/2016 09:07 PM, J. Eric Wonderley wrote:
> It defaults to 4k:
> mmlsfs testbs8M -i
> flag value
description
> ------------------- ------------------------
> -----------------------------------
> -i 4096 Inode size
in bytes
>
> I think you can make as small as 512b. Gpfs will
store very small
> files in the inode.
>
> Typically you want your average file size to be your
blocksize and your
> filesystem has one blocksize and one inodesize.
The files are not small, but around 20 MB on average.
So I calculated with IBM that a 1 MB or 2 MB block size
is best.
But I'm not sure if it's better to use a smaller block
size for the
metadata.
The file system is not that large (400 TB) and will hold
backup data
from CommVault.
Stef
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