Re: access to hard drives is "blocked" by writes to a flash drive

----- Original Message ----- 
From: "Ian Lepore" <ian_at_FreeBSD.org>
To: "Poul-Henning Kamp" <phk_at_phk.freebsd.dk>
Cc: "deeptech71" <deeptech71_at_gmail.com>; <freebsd-current_at_FreeBSD.org>; "Peter Jeremy" <peter_at_rulingia.com>
Sent: Sunday, March 03, 2013 1:54 PM
Subject: Re: access to hard drives is "blocked" by writes to a flash drive


> On Sun, 2013-03-03 at 13:35 +0000, Poul-Henning Kamp wrote:
>> Content-Type: text/plain; charset=ISO-8859-1
>> --------
>> In message <1362317291.1195.216.camel_at_revolution.hippie.lan>, Ian Lepore writes
>> :
>> 
>> >I run into this behavior all the time too, mostly on arm systems that
>> >have an sd card or usb thumb driver as their main/only drive.
>> 
>> This is really a FAQ and I belive I have answered it N times already:
>> 
>> There are, broadly speaking, two classes of flash-storage: "Camera-grade"
>> and "the real thing".
>> 
>> "Camera-grade" have a very limited "Flash adaptation layer" which typically
>> only can hold one flash-block open for writing at a time, and is typically
>> found in CF and SD cards, USB sticks etc.
>> 
>> Some of them gets further upset if the filesystem is not the FAT they
>> expect, because they implement "M-Systems" (patented) trick with monitoring
>> block deletes in FAT to simulate a TRIM facility.
>> 
>> A number of products exist with such designs, typically a CF-style, is
>> put behind a SATA-PATA bridge and sold as 2.5" SSD SATA devices.
>> "Transcend" have done this for instance.
>> 
>> If you use this class of devices for anything real, gstat will show
>> you I/O write-times of several seconds in periodic pile-ups, even
>> 100 seconds if you are doing something heavy.
>> 
>> For various reasons (see: Lemming-syncer) FreeBSD will block all I/O
>> traffic to other disks too, when these pileups gets too bad.
> 
> Hmmm, so the problem has been known and unfixed for 10 years.  That's
> not encouraging.  One of the messages in the lemming-syncer mail thread
> might explain why I've been seeing this a lot lately in hobbyist work,
> but not so much at $work where we use sd cards heavily... we use very
> short syncer timeouts on SD and CF storage at $work:
> 
> kern.metadelay: 3
> kern.dirdelay: 4
> kern.filedelay: 5
> 
> I might play with similar settings on some of my arm boards here.

Interesting, are these relevant for all filesystems e.g. ZFS?

    Regards
    Steve

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