Re: raid6check extremely slow ?

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

On Mon, May 11, 2020 at 11:44:11PM +0100, Peter Grandi wrote:
> >>> With lock / unlock, I get around 1.2MB/sec per device
> >>> component, with ~13% CPU load.  Wihtout lock / unlock, I get
> >>> around 15.5MB/sec per device component, with ~30% CPU load.
> 
> >> [...] we still need to avoid race conditions. [...]
> 
> Not all race conditions are equally bad in this situation.
> 
> > 1. Per your previous reply, only call raid6check when array is
> > RO, then we don't need the lock.
> > 2. Investigate if it is possible that acquire stripe_lock in
> > suspend_lo/hi_store [...]
> 
> Some other ways could be considered:
> 
> * Read a stripe without locking and check it; if it checks good,
>   no problem, else either it was modified during the read, or it
>   was faulty, so acquire a W lock, reread and recheck it (it
>   could have become good in the meantime).
> 
>   The assumption here is that there is a modest write load from
>   applications on the RAID set, so the check will almost always
>   succeed, and it is worth rereading the stripe in very rare
>   cases of "collisions" or faults.
> 
> * Variants, like acquiring a W lock (if possible) on the stripe
>   solely while reading it ("atomic" read, which may be possible
>   in other ways without locking) and then if check fails we know
>   it was faulty, so optionally acquire a new W lock and reread
>   and recheck it (it could have become good in the meantime).
> 
>   The assumption here is that the write load is less modest, but
>   there are a lot more reads than writes, so a W lock only
>   during read will eliminate the rereads and rechecks from
>   relatively rare "collisions".

The locking method was suggested by Neil,
I'm not aware of other methods.

About the check -> maybe lock -> re-check,
it is a possible workaround, but I find it
a bit extreme.

In any case, we should keep it in mind.

bye,

pg
 
> The case where there is at the same time a large application
> write load on the RAID set and checking at the same time is hard
> to improve and probably eliminating rereads and rechecks by just
> acquiring the stripe W lock for the whole duration of read and
> check.

-- 

piergiorgio