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

[Peter Jeremy]

On 2013-Mar-02 18:29:54 +0100, deeptech71 <deeptech71 at gmail.com> wrote:

>When one of my flash drives is being heavily written to; typically by
>``svn update'' on /usr/src, located on the flash drive; the following
>can be said about filesystem behavior:
>
>- ``svn update'' seems to be able to quickly update a bunch of files,
>   but is then unable to continue for a period of time. This behavior
>   is cyclical, and cycles several times, depending on the amount of
>   updating work to be done for a particular run of ``svn update''.

This sounds like normal flash behaviour:  You can only write to erased
blocks.  The SSD firmware attempts to keep a free pool of erased blocks
but if you write too fast, you empty the free pool and need to wait for
the wear-levelling algorithm to move blocks around and erase them.

Enabling TRIM (the '-t' flag on tunefs) will help if the drive supports
TRIM (if it doesn't, it'll probably just lockup).  Otherwise, you need
to either put up with it or upgrade to a better SSD.

I run into this regularly with the low-end SuperTalent drive in my
Netbook but have never seen it with the OCZ Agility4 that I use for
L2ARC in my fileserver.

-- 
Peter Jeremy
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