[lustre-discuss] Avoiding system cache when using ssd pfl extent

[Patrick Farrell]

Well, you could use two file descriptors, one for O_DIRECT one otherwise. 🙂

SSD is a fast medium but my instinct is the desirability of having data in RAM is much more about I/O pattern and hard to optimize for in advance - Do you read the data you wrote?  (Or read data repeatedly?)

In any case, there's no mechanism today.  It's also relatively marginal if we're just doing buffered I/O then forcing the data out - it will reduce memory usage but it won't improve performance.

-Patrick

________________________________
From: John Bauer <bauerj at iodoctors.com>
Sent: Thursday, May 19, 2022 1:16 PM
To: Patrick Farrell <pfarrell at ddn.com>; lustre-discuss at lists.lustre.org <lustre-discuss at lists.lustre.org>
Subject: Re: [lustre-discuss] Avoiding system cache when using ssd pfl extent


Pat,

No, not in  general.  It just seems that if one is storing data on an SSD it should be optional to have it not stored in memory ( why store in 2 fast mediums ).

O_DIRECT is not of value as that would apply to all extents, whether on SSD on HDD.   O_DIRECT on Lustre has been problematic for me in the past, performance wise.

John

On 5/19/22 13:05, Patrick Farrell wrote:
No, and I'm not sure I agree with you at first glance.

Is this just generally an idea that data stored on SSD should not be in RAM?  If so, there's no mechanism for that other than using direct I/O.

-Patrick
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From: lustre-discuss <lustre-discuss-bounces at lists.lustre.org><mailto:lustre-discuss-bounces at lists.lustre.org> on behalf of John Bauer <bauerj at iodoctors.com><mailto:bauerj at iodoctors.com>
Sent: Thursday, May 19, 2022 12:48 PM
To: lustre-discuss at lists.lustre.org<mailto:lustre-discuss at lists.lustre.org> <lustre-discuss at lists.lustre.org><mailto:lustre-discuss at lists.lustre.org>
Subject: [lustre-discuss] Avoiding system cache when using ssd pfl extent

When using PFL, and using an SSD as the first extent, it seems it would
be advantageous to not have that extent's file data consume memory in
the client's system buffers.  It would be similar to using O_DIRECT, but
on a per-extent basis.  Is there a mechanism for that already?

Thanks,

John

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