Re: [PATCH] bcache: back to cache all readahead I/Os

[Coly Li <colyli@suse.de>]

On 2020/1/14 8:34 下午, Nix wrote:
> On 6 Jan 2020, Eric Wheeler spake thusly:
> 
>> On Sat, 4 Jan 2020, Coly Li wrote:
>>
>>> In year 2007 high performance SSD was still expensive, in order to
>>> save more space for real workload or meta data, the readahead I/Os
>>> for non-meta data was bypassed and not cached on SSD.
> 
> It's also because readahead data is more likely to be useless.
> 
>>> In now days, SSD price drops a lot and people can find larger size
>>> SSD with more comfortable price. It is unncessary to bypass normal
>>> readahead I/Os to save SSD space for now.
> 

Hi Nix,

> Doesn't this reduce the utility of the cache by polluting it with
> unnecessary content? It seems to me that we need at least a *litle*
> evidence that this change is beneficial. (I mean, it might be beneficial
> if on average the data that was read ahead is actually used.)
> 
> What happens to the cache hit rates when this change has been running
> for a while?
> 

I have two reports offline and directly to me, one is from an email
address of github and forwarded to me by Jens, one is from a China local
storage startup.

The first report complains the desktop-pc benchmark is about 50% down
and the root cause is located on commit b41c9b0 ("bcache: update
bio->bi_opf bypass/writeback REQ_ flag hints").

The second report complains their small file workload (mixed read and
write) has around 20%+ performance drop and the suspicious change is
also focused on the readahead restriction.

The second reporter verifies this patch and confirms the performance
issue has gone. I don't know who is the first report so no response so far.

I don't have exact hit rate number because the reporter does not provide
(BTW, because the readahead request is bypassed, I feel the hit rate
won't count on them indeed). But from the reports and one verification,
IMHO this change makes sense.

Thanks.

-- 

Coly Li