Re: [gentoo-user] {OT} SSD instead of RAID1?

[Florian Philipp]

Grant schrieb:
>>> ... What if I bought a low-price/low-capacity SSD drive for each
>>> of these systems, installed the system essentials on them, and used my
>>> existing high-capacity HD drives for data storage?  Would each system
>>> keep running if the HDs died?  If so, I think that would offer as good
>>> or better system reliability than RAID1.  What do you think?
>> You don't need to buy SSD "drives" - instead you could use CF cards and a
>> cheap adaptor. These are commensurate in capacity & cost with USB flash
>> drives (4gig, maybe 16gig?), but CF cards "talk EIDE" and you can get cheap
>> pin-convertors allowing you to connect them to EIDE cables and treat them
>> like a hard-drive.
>
> Aren't CF cards much slower than SSD drives and HD drives?
>

Yep, especially the cheap ones which do not support DMA, just PIO. But
this is not necessarily a problem: After starting all services etc.
there will be very few reads on stuff like /etc and /usr. Just make sure
to put all directories to which you write (parts of /var like /var/log
and the several tmp directories) on an HDD, NFS or tmpfs. Of course,
this all depends on your usage patterns and how much RAM you have.

If you really need to write to the CFDisk, make sure to buy one with DMA
support (and no, the label "super fast" which is regularly found on
these things does not necessarily mean that it supports DMA).

One drawback of this configuration: You can never use swap - never!
Neither on the HDD (there is a high chance that the system would crash
when the HDD fails) nor on the (cheap) SSD/flash drive (the drive would
wear down, removing any advantage you tried to gain).

>> I know of these used in Asterisk based PABX systems & PoS tills with the
>> expectation that they're more reliable than disks, and have read statements
>> by people deploying quantities of such machines that they've never had a
>> failure in years of use.
>
> I like the sound of that.

Where I work, we have a System-on-a-Chip (SoC) NAS. Albeit being the
second most powerful machine we have in our server room (quad core CPU,
lots of RAM, three redundant power supplies and a good dozen HDDs), the
OSS itself resides on a removable card not bigger than my thumb.

>
>> I don't know how that really compares to RAID 1 - if you use hardware RAID
>> (and you can get hardware SATA controllers for £50 these days) then you can
>> assign a hot-spare, and hot-swap a replacement drive with zero downtime.
>> With hardware RAID you can still boot if one of the drives fails, but you do
>> add the controller as a potential point-of-failure.
>
> Would the system keeping running if I used a CF or SSD for the system
> install and the HD drive died?
>
> - Grant
>