[support-gang] Regarding temporary "pauses", perhaps due to using an SD card with an XO

[Yioryos Asprobounitis]

--- On Sat, 10/13/12, Mikus Grinbergs <mikus at bga.com> wrote:

> From: Mikus Grinbergs <mikus at bga.com>
> Subject: [support-gang] Regarding temporary "pauses", perhaps due to using an SD card with an XO
> To: "Community Support Volunteers -- who help respond to "help AT laptop.org"" <support-gang at lists.laptop.org>
> Cc: "OLPC Devel" <Devel at lists.laptop.org>
> Date: Saturday, October 13, 2012, 3:46 PM
> Lately there has been discussion
> about using an SD card to expand the "storage" capability of
> the XO.  [For instance, discussion of when an XO
> functions as a server, having it keep its database on an SD
> card.]
> 
> What I want to mention is that I experience occasional
> "pauses" on my XO systems (each has a "permanent" SD card,
> on which I keep Activities, Linux executables, etc.). 
> I do not know what causes these "pauses", but my best guess
> is the following (particularly since "pauses" often occur
> during a CLI command that is "massively" accessing the SD
> card).
> 
> My guess:   SD cards have an "embedded chip"
> which is responsible for "wear levelling" the electronic
> components used to keep the data.  It does this by
> internally 'rearranging' "which data block is physically
> kept where".  I think this 'rearranging' action by the
> "embedded chip" gets launched (when some 'internal' SD card
> thresholds are met) during an I/O access by the (XO) host --
> and this "rearranging" results in that host's I/O access
> being DELAYED until the "embedded chip" is done.
> 
> 
> I have not compared the "performance" (from the user's point
> of view) of running an Activity "from the SD card" vs.
> running it "from the XO".
> But I do have the impression that the duration of the
> occasional "pauses" which I as a user notice while running
> "with the SD card" have gotten LONGER as my XOs have
> migrated to using ever-larger SD cards.
> 
> mikus

Hi Mikus,
I do too experience these pauses with *specific* SDcards.
You may remember a relevant discussion back in 3-4/2011 on devel.
I would think that bigger cards may have bigger erase blocks and if they need to move 8 or 16MB blocks because a 6kb file cross that boundary things may "pause" on the XO, particularly if the card has limited ability (less than 5) of simultaneously open blocks.
You may want to use flashbench [ http://lists.laptop.org/pipermail/devel/2011-April/031677.html ] to check and optimize your card.
Further references (if you do not have them handy)
https://lwn.net/Articles/428584/
https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey?action=show&redirect=WorkingGroups%2FKernelConsolidation%2FProjects%2FFlashCardSurvey 

HTH

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