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List:       linux-bcache
Subject:    Re: A note and a question on discarding, from a novice bcache user
From:       Kai Krakow <kai () kaishome ! de>
Date:       2021-03-25 13:57:10
Message-ID: CAC2ZOYvoVfpNh-4hcYxPJkgA9kkK2Scbqg_ce2LO+Xk7D-6drQ () mail ! gmail ! com
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Hello Martin!

Am Do., 25. M=C3=A4rz 2021 um 14:06 Uhr schrieb Martin Kennedy <hurricos@gm=
ail.com>:
>
> I don't think the garbage collection performed by bcache actually
> sends TRIMs to the SSD unless you turn on discard. Without sending
> TRIMs you cannot expect a cheap SSD (or pair of cheap ones in RAID1)
> to keep up performance.
>
> After some small load over the month past, I've done another test to
> verify that performance has not degraded:

If you want to make use of trimmed space without using online discard,
your best chance is to create a blank partition on the caching device
after the bcache partition, blkdiscard that, and never touch it. Now
re-create bcache in the first partition. The blank partition should be
around 10-20% of your device size. This way, the SSD firmware can
still do background wear-leveling by swapping flash pages with the
untouched partition and do background gc/erase. This will keep bcache
latency low, and performance should be stable.
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