On 26/10/16 23:31, Vojtech Pavlik wrote:
I believe your faith in SSDs is somewhat misplaced, they do not so
infrequently die ahead of their SMART announcement and if they do, they
don't just get bad sectors, the whole device is gone.
In my experience they are extremely reliable, compared to traditional
drives, and much faster, which increases their reliability because a
rebuild/restore is much faster.
And of course I have redundant backups, stored in systems with
significantly distinct designs, locations, access controls, etc.
In case you want to protect your data, either use a RAID for your cache
devices, too, use it in write through mode, or in write-back mode with
zero dirty data target.
Ok, I'm not sure what this means, but it sounds like it is something I
might want to use. Have you got a link for this?
I am making assumptions about the ordering of writes that RAID5
makes, and will post to the appropriate list about that, with the
possibility of another option. However, I also note that bcache
"optimises" sequential writes directly to the underlying device:
In case you're using mdraid for the RAID part on a reasonably recent
Linux kernel, there is no write hole. Linux mdraid implements barriers
properly even on RAID5, at the cost of performance - mdraid waits for a
barrier to complete on all drives before submitting more i/o.
Any journalling, log or cow filesystem that relies on i/o barriers for
consistency will be consistent in Linux even on mdraid RAID5.
Ok, wow, I did not know this. Again, have you got a link to any
documentation about this. Unfortunately these kind of low-level systems
tend to be quite hard to find information about...
Using bcache to accelerate a RAID using a SSD is a fairly common use
case. What you're asking for can likely be achieved by:
echo writeback > cache_mode
echo 0 > writeback_percent
echo 10240 > writeback_rate
echo 5 > writeback_delay
echo 0 > readahead
echo 0 > sequential_cutoff
echo 0 > cache/congested_read_threshold_us
echo 0 > cache/congested_write_threshold_us
This is what I use personally on my system with success.
Thanks, I'll look at this. Genuinelly a much more helpful response than
I could ever have hoped for ;-)
James
--
To unsubscribe from this list: send the line "unsubscribe linux-bcache" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
