On Sep 26, 2012, at 4:28 AM, Sašo Kiselkov <skiselkov...@gmail.com> wrote:

> On 09/26/2012 01:14 PM, Edward Ned Harvey
> (opensolarisisdeadlongliveopensolaris) wrote:
>>> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
>>> boun...@opensolaris.org] On Behalf Of Jim Klimov
>>> Got me wondering: how many reads of a block from spinning rust
>>> suffice for it to ultimately get into L2ARC? Just one so it
>>> gets into a recent-read list of the ARC and then expires into
>>> L2ARC when ARC RAM is more needed for something else, 
>> Correct, but not always sufficient.  I forget the name of the parameter, but 
>> there's some rate limiting thing that limits how fast you can fill the 
>> L2ARC.  This means sometimes, things will expire from ARC, and simply get 
>> discarded.
> The parameters are:
> *) l2arc_write_max (default 8MB): max number of bytes written per
>    fill cycle

It should be noted that this level was perhaps appropriate 6 years
ago, when L2ARC was integrated and given the SSDs available at the
time, but is well below reasonable settings for high speed systems or
modern SSDs. It is probably not a bad idea to change the default to 
reflect more modern systems, thus avoiding surprises.
 -- richard

> *) l2arc_headroom (default 2x): multiplies the above parameter and
>    determines how far into the ARC lists we will search for buffers
>    eligible for writing to L2ARC.
> *) l2arc_feed_secs (default 1s): regular interval between fill cycles
> *) l2arc_feed_min_ms (default 200ms): minimum interval between fill
>    cycles
> Cheers,
> --
> Saso
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illumos Day & ZFS Day, Oct 1-2, 2012 San Fransisco 

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