Hi Marion,
I do not fully understand how to "mount flash OSTs on a metadata server"
- You have a couple of SSDs, you assemble these into on block device and format it with "mkfs.lustre
--ost ..." ? And then mount it just as any other OST?
- PFL then puts the first 64k on these OSTs and the rest of all files on the
HDD-based OSTs?
So, no magic on the MDS?
I'm asking because we are considering something similar, but we would not have these flash-OSTs in the
MDS-hardware but on separate OSS servers.
Regards,
Thomas
On 23/02/2022 04.35, Marion Hakanson via lustre-discuss wrote:
Hi again,
[email protected] said:
I was thinking that DoM is built in feature and it can be enabled/disabled
online for a certain directories. What do you mean by reformat to converting
to DoM (or away from it). I think just Metadata target size is important.
When we first turned on DoM, it's likely that our Lustre system was old
enough to need to be reformatted in order to support it. Our flash
storage RAID configuration also needed to be expanded, but the system
was not yet in production so a reformat was no big deal at the time.
So perhaps your system will not be subject to this requirement (other
than expanding your MDT flash somehow).
[email protected] said:
I also thought creating flash OST on metadata server. But I was not sure what
to install on metadata server for this purpose. Can Metadata server be an OSS
server at the same time? If it is possible I would prefer flash OST on
Metadata server instead of DoM. Because Our metadata target size is small, it
seems I have to do risky operations to expand size.
Yes, our metadata servers are also OSS's at the same time. The flash
OST's are separate volumes (and drives) from the MDT's, so less scary (:-).
[email protected] said:
imho, because of the less RPC traffic DoM shows more performance than flash
OST. Am I right?
The documentation does say there that using DoM for small files will produce
less RPC traffic than using OST's for small files.
But as I said earlier, for us, the amount of flash needed to support DoM
was a lot higher than with the flash OST approach (we have a high percentage,
by number, of small files).
I'll also note that we had a wish to mostly "set and forget" the layout
for our Lustre filesystem. We have not figured out a way to predict
or control where small files (or large ones) are going to end up, so
trying to craft optimal layouts in particular directories for particular
file sizes has turned out to not be feasible for us. PFL has been a
win for us here, for that reason.
Our conclusion was that in order to take advantage of the performance
improvements of DoM, you need enough money for lots of flash, or you need
enough staff time to manage the DoM layouts to fit into that flash.
We have neither of those conditions, and we find that using PFL and
flash OST's for small files is working very well for us.
Regards,
Marion
From: =?utf-8?B?VGFuZXIgS0FSQUfDlkw=?= <[email protected]>
To: Marion Hakanson <[email protected]>
CC: "[email protected]" <[email protected]>
Date: Tue, 22 Feb 2022 04:53:03 +0000
UNCLASSIFIED
Thank you for sharing your experience.
I was thinking that DoM is built in feature and it can be enabled/disabled
online for a certain directories. What do you mean by reformat to converting to
DoM (or away from it). I think just Metadata target size is important.
I also thought creating flash OST on metadata server. But I was not sure what
to install on metadata server for this purpose. Can Metadata server be an OSS
server at the same time? If it is possible I would prefer flash OST on Metadata
server instead of DoM. Because Our metadata target size is small, it seems I
have to do risky operations to expand size.
imho, because of the less RPC traffic DoM shows more performance than flash
OST. Am I right?
Best Regards;
From: Marion Hakanson <[email protected]>
Sent: Thursday, February 17, 2022 8:20 PM
To: Taner KARAGÖL <[email protected]>
Cc: [email protected]
Subject: Re: [lustre-discuss] How to speed up Lustre
We started with DoM on our new Lustre system a couple years ago.
- Converting to DoM (or away from it) is a full-reformat operation.
- DoM uses a fixed amount of metadata space (64k minimum for us) for every
file, even those smaller than 64k.
Basically, DoM uses a lot of flash metadata space, more than we planned for,
and more than we could afford.
We ended up switching to a PFL arrangement, where the first 64k lives on flash
OST's (mounted on our metadata servers), and the remainder of larger files
lives on HDD OST's. This is working very well for our small-file workloads,
and uses less flash space than the DoM configuration did.
Since you don't already have DoM in effect, it may be possible that you could add flash
OST's, configure a PFL, and then use "lfs migrate" to re-layout existing files
into the new OST's. Your mileage may vary, so be safe!
Regards,
Marion
On Feb 14, 2022, at 03:32, Taner KARAGÖL via lustre-discuss
<[email protected]<mailto:[email protected]>> wrote:

UNCLASSIFIED
Hi Everybody;
We have a performance problem with small files on our HPC system (120 compute
nodes). Our all OSS targets are classic spinning HDDs. To speed up, I want to
configure Data on Metadata. Our metadata target has SDD disks.
Underlying file systems are ZFS (for OSS and Meta)
Lustre version: 2.12.5
ZFS version: .0.7.13
Our Lustre file system size is 720TB (2 OSS servers, 1 enclosure with 6
zpools), Metadata file system size is 2.1TB(1 enclosure and 1 metadata target).
What is your opinions to speed up this setup? I want to configure DoM but I am
concerning about Metadata size. My questions:
1. How can I increase Medatadata size? Metadata enclosure has a empty
slots. Is there a way to increase size online/offline?
2. Is it possible to migrate big files from DoM to OSS targets completely?
Off course online migration. (So I think I can free Metadata for new small
files).
Best Regards;
Taner
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