On Mon, May 15, 2017 at 09:49:38PM +0200, Kai Krakow wrote: > > > It's worth noting also that on average, COW filesystems like BTRFS > > (or log-structured-filesystems will not benefit as much as > > traditional filesystems from SSD caching unless the caching is built > > into the filesystem itself, since they don't do in-place rewrites (so > > any new write by definition has to drop other data from the cache). > > Yes, I considered that, too. And when I tried, there was almost no > perceivable performance difference between bcache-writearound and > bcache-writeback. But the latency of performance improvement was much > longer in writearound mode, so I sticked to writeback mode. Also, > writing random data is faster because bcache will defer it to > background and do writeback in sector order. Sequential access is > passed around bcache anyway, harddisks are already good at that.
Let me add my 2 cents. bcache-writearound does not cache writes on SSD, so there are less writes overall to flash. It is said to prolong the life of the flash drive. I've recently switched from bcache-writeback to bcache-writearound, because my SSD caching drive is at the edge of it's lifetime. I'm using bcache in following configuration: https://enotty.pipebreaker.pl/dżogstaff/2016.05.25-opcja2.svg My SSD is Samsung SSD 850 EVO 120GB, which I bought exactly 2 years ago. Now, according to http://www.samsung.com/semiconductor/minisite/ssd/product/consumer/850evo.html 120GB and 250GB warranty only covers 75 TBW (terabytes written). My drive has # smartctl -a /dev/sda | grep LBA 241 Total_LBAs_Written 0x0032 099 099 000 Old_age Always - 136025596053 which multiplied by 512 bytes gives 69.6 TB. Close to 75TB? Well… [35354.697513] sd 0:0:0:0: [sda] tag#19 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE [35354.697516] sd 0:0:0:0: [sda] tag#19 Sense Key : Medium Error [current] [35354.697518] sd 0:0:0:0: [sda] tag#19 Add. Sense: Unrecovered read error - auto reallocate failed [35354.697522] sd 0:0:0:0: [sda] tag#19 CDB: Read(10) 28 00 0c 30 82 9f 00 00 48 00 [35354.697524] blk_update_request: I/O error, dev sda, sector 204505785 Above started appearing recently. So, I was really suprised that: - this drive is only rated for 120 TBW - I went through this limit in only 2 years The workload is lightly utilised home server / media center. -- Tomasz Torcz Only gods can safely risk perfection, xmpp: zdzich...@chrome.pl it's a dangerous thing for a man. -- Alia -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
