1 |
On 22/02/2014 11:41, J. Roeleveld wrote: |
2 |
> On Sat, February 22, 2014 06:27, Facundo Curti wrote: |
3 |
>> Hi all. I'm new in the list, this is my third message :) |
4 |
>> First at all, I need to say sorry if my english is not perfect. I speak |
5 |
>> spanish. I post here because gentoo-user-es it's middle dead, and it's a |
6 |
>> great chance to practice my english :) Now, the problem. |
7 |
> |
8 |
> First of all, there are plenty of people here who don't have English as a |
9 |
> native language. Usually we manage. :) |
10 |
> |
11 |
>> I'm going to get a new PC with a disc SSD 120GB and another HDD of 1TB. |
12 |
>> But in a coming future, I want to add 2 or more disks SSD. |
13 |
>> |
14 |
>> Mi idea now, is: |
15 |
>> |
16 |
>> Disk HHD: /dev/sda |
17 |
>> /dev/sda1 26GB |
18 |
>> /dev/sda2 90GB |
19 |
>> /dev/sda3 904GB |
20 |
>> |
21 |
>> Disk SSD: /dev/sdb |
22 |
>> /dev/sdb1 26GB |
23 |
>> /dev/sdb2 90GB |
24 |
>> /dev/sdb3 4GB |
25 |
>> |
26 |
>> And use /dev/sdb3 as swap. (I will add more with another SSD in future) |
27 |
>> /dev/sda3 mounted in /home/user/data (to save data unused) |
28 |
> |
29 |
> Why put the swap on the SSD? |
30 |
> |
31 |
>> And a RAID 1 with: |
32 |
>> md0: sda1+sdb1 / |
33 |
>> md1: sda2+sdb2 /home |
34 |
>> |
35 |
>> (sda1 and sda2 will be made with the flag: write-mostly. This is useful |
36 |
>> for |
37 |
>> disks slower). |
38 |
>> In a future, I'm going to add more SSD's on this RAID. My idea is the |
39 |
>> fastest I/O. |
40 |
>> |
41 |
>> Now. My problem/question is: |
42 |
>> Following the gentoo's |
43 |
>> doc<http://www.gentoo.org/doc/es/gentoo-x86+raid+lvm2-quickinstall.xml>, |
44 |
>> it says I need to put the flag --metadata=0.9 on the RAID. My question is |
45 |
>> ¿This will make get off the performance?. |
46 |
|
47 |
It has no impact on performance. |
48 |
|
49 |
> |
50 |
> metadata=0.9 might be necessary for the BIOS of your computer to see the |
51 |
> /boot partition. If you use an initramfs, you can use any metadata you |
52 |
> like for the root-partition. |
53 |
|
54 |
The BIOS should not care at all as it is charged only with loading code |
55 |
from the MBR. However, if the intention is to use grub-0.97 then the |
56 |
array hosting the filesystem containing /boot should: |
57 |
|
58 |
* use RAID-1 |
59 |
* use the 0.90 superblock format |
60 |
|
61 |
That way, grub-0.97 can read the filesystem from either block device |
62 |
belonging to the array. Doing it any other way requires a bootloader |
63 |
that specifically understands md (such as grub2). |
64 |
|
65 |
There's also the neat trick of installing grub to all disks belonging to |
66 |
the array for bootloader redundancy. However, I'm not entirely sure that |
67 |
Code Listing 2.35 in the Gentoo doc is correct. Given that particular |
68 |
example, I would instead do it like this:- |
69 |
|
70 |
grub> device (hd0) /dev/sda |
71 |
grub> root (hd0,0) |
72 |
grub> setup (hd0) |
73 |
grub> device (hd0) /dev/sdb |
74 |
grub> root (hd0,0) |
75 |
grub> setup (hd0) |
76 |
|
77 |
The idea there is that, should it ever be necessary to boot from the |
78 |
second disk, the disk in question would be the one enumerated first by |
79 |
the BIOS (mapping to hd0 in grub). Therefore, grub should be installed |
80 |
in that context across all disks. It should not be allowed to boot from |
81 |
any given drive and subsequently try to access >=(hd1). |
82 |
|
83 |
With grub2, it's a little easier because it is only necessary to run |
84 |
grub-install on each of the drives: |
85 |
|
86 |
# grub-install /dev/sda |
87 |
# grub-install /dev/sdb |
88 |
|
89 |
> |
90 |
>> I only found this |
91 |
>> document<https://raid.wiki.kernel.org/index.php/RAID_superblock_formats#The_version-0.90_Superblock_Format>. |
92 |
>> This says the difference, but nothing about performance and |
93 |
>> advantages/disadvantages. |
94 |
|
95 |
The 0.90 superblock format is subject to specific limitations that are |
96 |
clearly described by that page. For example, it is limited to 28 devices |
97 |
in an array, with each device being limited to 2TB in size. |
98 |
|
99 |
Also, the 0.90 format will cause issues in certain setups because of the |
100 |
way that it places its metadata at the end of the block device [1]. That |
101 |
said, the 0.90 format does allow for the kernel to construct the array |
102 |
without any intervention from userspace so it still has its uses. |
103 |
|
104 |
The 1.2 format positions the superblock 4KiB from the beginning of the |
105 |
device. Note that this has nothing at all to do with the data, which |
106 |
usually begins 1MiB in. If you run mdadm -E on a member of such an |
107 |
array, the offset will be reported as the "Data Offset". For example: |
108 |
|
109 |
Data Offset : 2048 sectors |
110 |
|
111 |
So, it's not a matter of alignment. Rather, the advantage of the 1.2 |
112 |
format is that it leaves a little space for bootloader code e.g. in case |
113 |
you want to create an array from whole disks rather than disk partitions. |
114 |
|
115 |
None of this matters to me so I tend to stick to the 1.1 format. It |
116 |
wouldn't actually make any difference to my particular use case. |
117 |
|
118 |
|
119 |
>> |
120 |
>> Another question is, ¿GRUB2 still unsupporting metadata 1.2? |
121 |
> |
122 |
> See reply from Canek. |
123 |
> |
124 |
>> In case that metadata get off performance, and GRUB2 doesn't support this. |
125 |
>> ¿Anyone knows how can I fix this to use metadata 1.2? |
126 |
>> |
127 |
>> I don't partitioned more, because I saw this unnecessary. I just need to |
128 |
>> separate /home in case I need to format the system. But if I need to |
129 |
>> separate /boot to make it work, I don't have problems doing that. |
130 |
>> |
131 |
>> But of course, /boot also as RAID... |
132 |
> |
133 |
> /boot seperate as RAID-1 and metadata=0.9 and you are safe. |
134 |
> |
135 |
>> ¿Somebody have any ideas to make it work? |
136 |
> |
137 |
> It is similar to what I do, except I don't have SSDs in my desktop. |
138 |
> |
139 |
> I have 2 partitions per disk: |
140 |
> 1 : /boot (mirrored, raid-1) |
141 |
> 2 : LVM (striped, raid-0) |
142 |
> All other partitions (root, /usr, /home, ....) are in the LVM. |
143 |
> |
144 |
> I use striping for performance reasons for files I currently work with. |
145 |
> All important data is stored and backed up on a server. |
146 |
> |
147 |
> For this, an initramfs is required with support for mdraid and lvm. |
148 |
|
149 |
Some people would make a point of avoiding the initramfs requirement by |
150 |
just having a relatively small root filesystem on RAID-1 with 0.90 |
151 |
metadata. Unfortunately, the UsrMove affair has now rendered this |
152 |
approach impractical because /usr can no longer be separate. It's still |
153 |
an option, though. |
154 |
|
155 |
|
156 |
--Kerin |
157 |
|
158 |
[1] https://savannah.gnu.org/bugs/?34250 |