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From: Matthew Wilcox <>
To: Ric Wheeler <>
Cc: Jaegeuk Kim <>,
	Bart Van Assche <>,,
	Linux FS Devel <>,
Subject: Re: [LSF/MM/BPF TOPIC] durability vs performance for flash devices (especially embedded!)
Date: Thu, 10 Jun 2021 18:06:50 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, Jun 10, 2021 at 12:22:40PM -0400, Ric Wheeler wrote:
> On 6/9/21 5:32 PM, Jaegeuk Kim wrote:
> > On Wed, Jun 9, 2021 at 11:47 AM Bart Van Assche <
> > <>> wrote:
> > 
> >     On 6/9/21 11:30 AM, Matthew Wilcox wrote:
> >     > maybe you should read the paper.
> >     >
> >     > " Thiscomparison demonstrates that using F2FS, a flash-friendly file
> >     > sys-tem, does not mitigate the wear-out problem, except inasmuch asit
> >     > inadvertently rate limitsallI/O to the device"
> > 
> > 
> > Do you agree with that statement based on your insight? At least to me, that
> > paper is missing the fundamental GC problem which was supposed to be
> > evaluated by real workloads instead of using a simple benchmark generating
> > 4KB random writes only. And, they had to investigate more details in FTL/IO
> > patterns including UNMAP and LBA alignment between host and storage, which
> > all affect WAF. Based on that, the point of the zoned device is quite promising
> > to me, since it can address LBA alignment entirely and give a way that host
> > SW stack can control QoS.
> Just a note, using a pretty simple and optimal streaming write pattern, I
> have been able to burn out emmc parts in a little over a week.
> My test case creating a 1GB file (filled with random data just in case the
> device was looking for zero blocks to ignore) and then do a loop to cp and
> sync that file until the emmc device life time was shown as exhausted.
> This was a clean, best case sequential write so this is not just an issue
> with small, random writes.

How many LBAs were you using?  My mental model of a FTL (which may
be out of date) is that it's essentially a log-structured filesystem.
When there are insufficient empty erase-blocks available, the device
finds a suitable victim erase-block, copies all the still-live LBAs into
an active erase-block, updates the FTL and erases the erase-block.

So the key is making sure that LBAs are reused as much as possible.
Short of modifying a filesystem to make this happen, I force it by
short-stroking my SSD.  We can model it statistically, but intuitively,
if there are more "live" LBAs, the higher the write amplification and
wear on the drive will be because the victim erase-blocks will have
more live LBAs to migrate.

This is why the paper intrigued me; it seemed like they were rewriting
a 100MB file in place.  That _shouldn't_ cause ridiculous wear, unless
the emmc device was otherwise almost full.

  reply	other threads:[~2021-06-10 17:07 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-09 10:53 [LSF/MM/BPF TOPIC] durability vs performance for flash devices (especially embedded!) Ric Wheeler
2021-06-09 18:05 ` Bart Van Assche
2021-06-09 18:30   ` Matthew Wilcox
2021-06-09 18:47     ` Bart Van Assche
2021-06-10  0:16       ` Damien Le Moal
2021-06-10  1:11         ` Ric Wheeler
2021-06-10  1:20       ` Ric Wheeler
2021-06-10 11:07         ` Tim Walker
2021-06-10 16:38           ` Keith Busch
     [not found]       ` <>
2021-06-10 16:22         ` Ric Wheeler
2021-06-10 17:06           ` Matthew Wilcox [this message]
2021-06-10 17:25             ` Ric Wheeler
2021-06-10 17:57           ` Viacheslav Dubeyko
2021-06-13 20:41 ` [LSF/MM/BPF TOPIC] SSDFS: LFS file system without GC operations + NAND flash devices lifetime prolongation Viacheslav Dubeyko

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