Re: Home Directory in SSD

Hi!

I see no issues against uses btrfs send/receive or rsync as backup from SSD to 
HDD. With btrfs send/receive I think after the initial sync it can even work 
quickly enough to have it run every 10 minutes or so. But if RAID allows 
write-mostly, why not. I think BTRFS RAID does not (yet).

Am Sonntag, 7. Februar 2016, 17:49:14 CET schrieb Matus UHLAR - fantomas:
> >> 2. using bcache, dm-cache or flashcache (cache HDD on SSD)
> >
> >If you use bcache with write caching you basically double your risk of data
> >loss.
> 
> double? explain, please.

If one of two disk fails => boom. Unless you use bcache not for caching 
writes, but even then I am not sure as it creates a new block device on top of 
the ssd and harddisk – I think it can at least lead to some filesystem 
inconsistencies. In case you use RAID 1 or regular rsync / btrfs send/receive 
one disk can fail and you are still good. So comparing these two approaches 
its even more than double the risk. Its about double the risk comparing with 
storing different data on SSD and HDD. You loose the data that the broken 
device stores, but with bcache you loose the data if any of the devices break.

> >And alternative is also using a hybrid harddisk that uses a little amount
> >of flash as caching. You have one device and its all transparent. But you
> >have lots of logic in firmware. Seagate does these, I am not sure whether
> >other vendors do these meanwhile as well.
> 
> hybrid HDD is very nice alternative, and in fact it is what I would
> like to "emulate" using either flash cache mechanism mentioned above.
> The main difference is that current hybrids only have 8GB of cache, so the
> speedup is not that noticeable.

Even though you have two parts in such a hybrid HDD that can fail, it is at 
least one device.


-- 
Martin