+- Forum (http://forum.xbian.org)
+-- Forum: Software (/forum-6.html)
+--- Forum: Installation (/forum-16.html)
+--- Thread: [IDEA] Psuedo Mirroring (/thread-2207.html)
Psuedo Mirroring - IriDium - 26th Mar, 2014 03:35 AM
Currently I have a 4Gb SD card and a 8Gb USB flash drive with Xbian booting off of the USB drive.
The SD card is only used to boot from, so the rest of the space is wasted.
Would it be possible to have a “mirror” function that would mirror the USB drive so that I have Xbian (and all updates) on both the USB and the SD card – so they are mirrored (to an extent) so if the USB fails I still have a working copy on the SD card (by just altering cmdline.txt)
Just a thought!
RE: Psuedo Mirroring - CurlyMo - 26th Mar, 2014 05:34 AM
The problem is that btrfs doesn't (normally) allow you to boot from a degraded mirror. If that's not a problem you can.
https://github.com/xbianonpi/xbian-package-initramfs/issues/18#issuecomment-20597109
Quote:I did some benchmarks and the results are as follows (SD card + USB stick):
Code:
random random
KB reclen write rewrite read reread read write
RAID1 : 51200 4 493 518 4578 4346 4440 649
RAID0 :
- USB : 51200 4 730 1092 3788 2979 3691 281
- SD : 51200 4 992 788 5558 4706 4974 732
-33% -35% +20% +50% +20% +130%
RAID1 : 51200 512 3748 4475 18705 11533 18672 4521
RAID0 :
- USB : 51200 512 3157 3488 23652 17815 24158 2918
- SD : 51200 512 4849 4182 17513 17537 17443 4198
+20% +28% +6% -35% +7% +55%
RAID1 : 51200 16384 5572 5456 25016 24991 24994 3247
RAID0 :
- USB : 51200 16384 4151 4492 27078 27682 27621 3114
- SD : 51200 16384 7632 9915 22041 22032 22050 9801
+31% +21% +13% +13% +13% +4%
The percentages were calculated like this:
Increase: How much faster than the slowest of the USB/SD
Decrease: How much slower than the slowest of the USB/SD
Conclusion:
- 4k write performance dropped by about 33%
- 4k read performance has increased for about 33%*
- All other IO show an increase of a about 20%*, except for the 512k reread.
*With the slowest device as the reference
I used a fairly slow and generic USB stick. However, the results show that the performance of the USB stick can be increased fairly well by a fast SD card. The ideal solution would of course be, adding a USB stick that is faster than the SD card so we get a overall performance gain instead of decrease.
The only problem is that you can't simply go back to the RAID0 situation. If you want to remove the USB drive from the mirror, you need to boot degraded mode.
RE: Psuedo Mirroring - mk01 - 26th Mar, 2014 01:17 PM
@IriDium
you want online mirror or offline mirror? cause offline mirror is simply a clone, or cp, or rsync, or what ever - so as we have xbianclone(copy) I would use this to make daily copy (actually if I implement incremental snapshots copy then we will even not push hardware by copying all content daily).
cron record would look like this
Code:
btrfs-auto-snapshot xbiancopy /dev/root /dev/mmcblk0p2online mirror you can do with sw raid, or with device mapper layer, or easiest with btrfs. but online multidisk solutions have some drawbacks (for user). for example writes. by default system will wait for all devices to finish a block write (so if you add slower device, you will slow down on writes to level of slowest device).