Gentoo Archives: gentoo-user

From: Grant <emailgrant@×××××.com>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] {OT} SSD instead of RAID1?
Date: Tue, 28 Jul 2009 17:52:26
In Reply to: Re: [gentoo-user] {OT} SSD instead of RAID1? by Florian Philipp
1 >>>> ... What if I bought a low-price/low-capacity SSD drive for each
2 >>>> of these systems, installed the system essentials on them, and used my
3 >>>> existing high-capacity HD drives for data storage?  Would each system
4 >>>> keep running if the HDs died?  If so, I think that would offer as good
5 >>>> or better system reliability than RAID1.  What do you think?
6 >>> You don't need to buy SSD "drives" - instead you could use CF cards and a
7 >>> cheap adaptor. These are commensurate in capacity & cost with USB flash
8 >>> drives (4gig, maybe 16gig?), but CF cards "talk EIDE" and you can get cheap
9 >>> pin-convertors allowing you to connect them to EIDE cables and treat them
10 >>> like a hard-drive.
11 >>
12 >> Aren't CF cards much slower than SSD drives and HD drives?
13 >>
14 >
15 > Yep, especially the cheap ones which do not support DMA, just PIO. But
16 > this is not necessarily a problem: After starting all services etc.
17 > there will be very few reads on stuff like /etc and /usr. Just make sure
18 > to put all directories to which you write (parts of /var like /var/log
19 > and the several tmp directories) on an HDD, NFS or tmpfs. Of course,
20 > this all depends on your usage patterns and how much RAM you have.
21 >
22 > If you really need to write to the CFDisk, make sure to buy one with DMA
23 > support (and no, the label "super fast" which is regularly found on
24 > these things does not necessarily mean that it supports DMA).
25 >
26 > One drawback of this configuration: You can never use swap - never!
27 > Neither on the HDD (there is a high chance that the system would crash
28 > when the HDD fails) nor on the (cheap) SSD/flash drive (the drive would
29 > wear down, removing any advantage you tried to gain).
30 >
31 >>> I know of these used in Asterisk based PABX systems & PoS tills with the
32 >>> expectation that they're more reliable than disks, and have read statements
33 >>> by people deploying quantities of such machines that they've never had a
34 >>> failure in years of use.
35 >>
36 >> I like the sound of that.
37 >
38 > Where I work, we have a System-on-a-Chip (SoC) NAS. Albeit being the
39 > second most powerful machine we have in our server room (quad core CPU,
40 > lots of RAM, three redundant power supplies and a good dozen HDDs), the
41 > OSS itself resides on a removable card not bigger than my thumb.
43 Is cost-savings the advantage of using CF instead of SSD? It sounds
44 like it might be wiser to spend a little more (low capacity SSD drives
45 are pretty cheap now) and have a real storage device that doesn't need
46 an adapter and is much faster, can swap, etc.
48 I bet I'm missing something though...?
50 - Grant
53 >>> I don't know how that really compares to RAID 1 - if you use hardware RAID
54 >>> (and you can get hardware SATA controllers for £50 these days) then you can
55 >>> assign a hot-spare, and hot-swap a replacement drive with zero downtime.
56 >>> With hardware RAID you can still boot if one of the drives fails, but you do
57 >>> add the controller as a potential point-of-failure.
58 >>
59 >> Would the system keeping running if I used a CF or SSD for the system
60 >> install and the HD drive died?
61 >>
62 >> - Grant


Subject Author
Re: [gentoo-user] {OT} SSD instead of RAID1? Stroller <stroller@××××××××××××××××××.uk>