我从AWS上的四个EBS快照创build了四个卷。 这四个卷组成了一个BTRFS多设备驱动器。 驱动器没有分区表，即我运行mkfs.btrfs /dev/xvdf /dev/xvdg等
创build卷时，请使用AWS CLI工具附加这些卷，然后运行btrfs device scan /dev/xvd{f,g,h,i} 。 这些设备有一个标签，所以挂载他们我运行（我试了几件事） mount -t btrfs -o ro,nospace_cache,clear_cache -L LABEL /tmp/dir 。 这个挂了很长时间，15-20分钟。 这些设备每个都是2TB。
当我在dmesg中看到下面的消息。 当dmesg挂起时，在dmesg中不会显示额外的输出。 我不知道是什么原因导致这种情况。
[ 3316.093665] blkfront: xvdf: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3316.124269] xvdf: unknown partition table [ 3316.430563] btrfs: device label BtrFcdData devid 1 transid 592370 /dev/xvdf [ 3316.432660] btrfs: device label BtrFcdData devid 1 transid 592370 /dev/xvdf [ 3317.067647] blkfront: xvdg: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3317.111985] xvdg: unknown partition table [ 3317.450861] btrfs: device label BtrFcdData devid 2 transid 592370 /dev/xvdg [ 3317.453036] btrfs: device label BtrFcdData devid 2 transid 592370 /dev/xvdg [ 3318.186270] blkfront: xvdh: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3318.232822] xvdh: unknown partition table [ 3319.025208] btrfs: device label BtrFcdData devid 3 transid 592370 /dev/xvdh [ 3319.027380] btrfs: device label BtrFcdData devid 3 transid 592370 /dev/xvdh [ 3320.067903] blkfront: xvdi: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3320.123265] xvdi: unknown partition table [ 3320.495547] btrfs: device label BtrFcdData devid 4 transid 592370 /dev/xvdi [ 3320.497922] btrfs: device label BtrFcdData devid 4 transid 592370 /dev/xvdi [ 3321.436803] btrfs: device label BtrFcdData devid 1 transid 592370 /dev/xvdf [ 3321.437252] btrfs: device label BtrFcdData devid 2 transid 592370 /dev/xvdg [ 3321.437641] btrfs: device label BtrFcdData devid 3 transid 592370 /dev/xvdh [ 3321.438040] btrfs: device label BtrFcdData devid 4 transid 592370 /dev/xvdi [ 3457.266257] blkfront: xvdf: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3457.315871] xvdf: unknown partition table [ 3457.862287] btrfs: device label BtrFcdData devid 1 transid 592370 /dev/xvdf [ 3457.864513] btrfs: device label BtrFcdData devid 1 transid 592370 /dev/xvdf [ 3458.413241] blkfront: xvdg: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3458.519586] xvdg: unknown partition table [ 3458.941200] btrfs: device label BtrFcdData devid 2 transid 592370 /dev/xvdg [ 3458.943531] btrfs: device label BtrFcdData devid 2 transid 592370 /dev/xvdg [ 3460.121281] blkfront: xvdh: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3460.198695] xvdh: unknown partition table [ 3460.805858] btrfs: device label BtrFcdData devid 3 transid 592370 /dev/xvdh [ 3460.808173] btrfs: device label BtrFcdData devid 3 transid 592370 /dev/xvdh [ 3461.146042] blkfront: xvdi: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; [ 3461.271872] xvdi: unknown partition table [ 3462.355409] btrfs: device label BtrFcdData devid 4 transid 592370 /dev/xvdi [ 3462.357822] btrfs: device label BtrFcdData devid 4 transid 592370 /dev/xvdi [ 3463.185250] btrfs: device label BtrFcdData devid 1 transid 592370 /dev/xvdf [ 3463.185669] btrfs: device label BtrFcdData devid 2 transid 592370 /dev/xvdg [ 3463.186098] btrfs: device label BtrFcdData devid 3 transid 592370 /dev/xvdh [ 3463.186471] btrfs: device label BtrFcdData devid 4 transid 592370 /dev/xvdi [ 3463.217317] btrfs: device label BtrFcdData devid 4 transid 592370 /dev/xvdi [ 3463.219230] btrfs: disabling disk space caching [ 3463.219233] btrfs: force clearing of disk cache [ 4930.891445] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 2 transid 27031 /dev/xvdb [ 4930.893685] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 2 transid 27031 /dev/xvdb [ 4930.949380] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 1 transid 27031 /dev/xvdc [ 4930.951209] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 1 transid 27031 /dev/xvdc [ 4937.133509] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 2 transid 27031 /dev/xvdb [ 4937.135297] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 2 transid 27031 /dev/xvdb [ 4937.135757] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 1 transid 27031 /dev/xvdc [ 4937.137575] btrfs: device fsid e512929a-72e4-4bf9-bf1d-c4744bb9cb06 devid 1 transid 27031 /dev/xvdc
编辑：
运行watch iostat -dk 2 -x 1显示了一些不同的数字，起初它在前两个设备上显示出高利用率，然后显示：
Device: rrqm/s wrqm/sr/sw/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util xvda 0.01 1.49 0.87 2.99 22.34 274.17 153.71 1.62 419.08 5.06 540.15 3.91 1.51 xvdd 0.00 0.00 0.02 0.00 0.07 0.00 7.91 0.00 0.41 0.41 0.00 0.41 0.00 xvdb 0.00 0.10 1.53 1.59 23.88 31.80 35.69 0.16 52.79 5.00 98.68 2.80 0.87 xvdc 0.00 0.08 1.58 1.37 24.99 27.43 35.49 0.25 84.70 5.21 176.74 3.45 1.02 xvdf 0.00 2.11 0.48 13.36 7.44 1325.75 192.64 2.25 162.64 5.33 168.27 2.18 3.01 xvdt 0.00 0.00 0.01 0.00 0.08 0.00 31.25 0.00 77.77 77.77 0.00 72.35 0.04 xvdy 0.00 0.00 0.00 0.00 0.03 0.00 30.32 0.00 102.43 102.43 0.00 101.05 0.02 xvdu 0.00 0.00 0.00 0.00 0.00 0.00 20.96 0.00 75.64 75.64 0.00 75.64 0.00 xvdo 0.00 0.00 0.00 0.00 0.01 0.00 16.84 0.00 38.53 38.53 0.00 38.53 0.00
这是syslog的一个片段：
Nov 23 17:35:01 ip-10-0-1-123 CRON[11806]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1) Nov 23 17:38:28 ip-10-0-1-123 kernel: [605946.625013] blkfront: xvdt: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; Nov 23 17:38:28 ip-10-0-1-123 kernel: [605946.732903] xvdt: unknown partition table Nov 23 17:38:28 ip-10-0-1-123 kernel: [605947.017249] btrfs: device label BtrFcdData devid 1 transid 616951 /dev/xvdt Nov 23 17:38:29 ip-10-0-1-123 kernel: [605947.036501] btrfs: device label BtrFcdData devid 1 transid 616951 /dev/xvdt Nov 23 17:38:30 ip-10-0-1-123 kernel: [605948.575945] blkfront: xvdy: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; Nov 23 17:38:30 ip-10-0-1-123 kernel: [605948.624099] xvdy: unknown partition table Nov 23 17:38:30 ip-10-0-1-123 kernel: [605948.944109] btrfs: device label BtrFcdData devid 2 transid 616951 /dev/xvdy Nov 23 17:38:30 ip-10-0-1-123 kernel: [605948.946964] btrfs: device label BtrFcdData devid 2 transid 616951 /dev/xvdy Nov 23 17:38:31 ip-10-0-1-123 kernel: [605949.783777] blkfront: xvdu: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; Nov 23 17:38:31 ip-10-0-1-123 kernel: [605949.839874] xvdu: unknown partition table Nov 23 17:38:32 ip-10-0-1-123 kernel: [605950.260981] btrfs: device label BtrFcdData devid 3 transid 616951 /dev/xvdu Nov 23 17:38:32 ip-10-0-1-123 kernel: [605950.263252] btrfs: device label BtrFcdData devid 3 transid 616951 /dev/xvdu Nov 23 17:38:33 ip-10-0-1-123 kernel: [605951.590060] blkfront: xvdo: barrier or flush: disabled; persistent grants: enabled; indirect descriptors: enabled; Nov 23 17:38:33 ip-10-0-1-123 kernel: [605951.662633] xvdo: unknown partition table Nov 23 17:38:33 ip-10-0-1-123 kernel: [605951.950477] btrfs: device label BtrFcdData devid 4 transid 616951 /dev/xvdo Nov 23 17:38:33 ip-10-0-1-123 kernel: [605951.953476] btrfs: device label BtrFcdData devid 4 transid 616951 /dev/xvdo Nov 23 17:38:35 ip-10-0-1-123 kernel: [605953.098657] btrfs: device label BtrFcdData devid 1 transid 616951 /dev/xvdt Nov 23 17:38:35 ip-10-0-1-123 kernel: [605953.099002] btrfs: device label BtrFcdData devid 2 transid 616951 /dev/xvdy Nov 23 17:38:35 ip-10-0-1-123 kernel: [605953.099292] btrfs: device label BtrFcdData devid 3 transid 616951 /dev/xvdu Nov 23 17:38:35 ip-10-0-1-123 kernel: [605953.099723] btrfs: device label BtrFcdData devid 4 transid 616951 /dev/xvdo Nov 23 17:38:35 ip-10-0-1-123 kernel: [605953.135590] btrfs: device label BtrFcdData devid 4 transid 616951 /dev/xvdo Nov 23 17:38:35 ip-10-0-1-123 kernel: [605953.137944] btrfs: disabling disk space caching Nov 23 17:38:35 ip-10-0-1-123 kernel: [605953.137948] btrfs: force clearing of disk cache Nov 23 17:38:36 ip-10-0-1-123 kernel: [605954.947400] btrfs: device label BtrFcdData devid 4 transid 616951 /dev/xvdo Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.868106] INFO: task mount:12477 blocked for more than 120 seconds. Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.871838] Not tainted 3.13.0-92-generic #139-Ubuntu Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.874879] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878538] mount D ffff88020fc13180 0 12477 7218 0x00000000 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878543] ffff88016d32bc18 0000000000000086 ffff88009bb8e000 0000000000013180 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878547] ffff88016d32bfd8 0000000000013180 ffff88009bb8e000 ffff88009bb8e000 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878550] ffff8800776ca870 ffff8800776ca878 ffffffff00000000 ffff8800776ca880 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878553] Call Trace: Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878562] [<ffffffff8172e2e9>] schedule+0x29/0x70 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878566] [<ffffffff81730f05>] rwsem_down_write_failed+0x115/0x230 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878571] [<ffffffff811dd0eb>] ? iput+0x3b/0x180 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878579] [<ffffffffa00b0000>] ? 0xffffffffa00affff Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878584] [<ffffffff81377da3>] call_rwsem_down_write_failed+0x13/0x20 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878602] [<ffffffffa00b51fa>] ? btrfs_mount+0x2ea/0x800 [btrfs] Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878605] [<ffffffff8173093d>] ? down_write+0x2d/0x30 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878609] [<ffffffff811c3c7e>] grab_super+0x2e/0xa0 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878611] [<ffffffff811c42f0>] sget+0x2d0/0x400 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878621] [<ffffffffa00b29a0>] ? btrfs_parse_early_options+0x1b0/0x1b0 [btrfs] Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878630] [<ffffffffa00b5328>] btrfs_mount+0x418/0x800 [btrfs] Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878634] [<ffffffff811735aa>] ? pcpu_alloc+0x6ca/0xa50 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878637] [<ffffffff8119ad33>] ? alloc_pages_current+0xa3/0x160 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878641] [<ffffffff811c5149>] mount_fs+0x39/0x1b0 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878644] [<ffffffff811e0917>] vfs_kern_mount+0x67/0x110 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878648] [<ffffffff811e31ef>] do_mount+0x25f/0xcd0 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878652] [<ffffffff81156dae>] ? __get_free_pages+0xe/0x50 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878655] [<ffffffff811e2e16>] ? copy_mount_options+0x36/0x170 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878658] [<ffffffff811e3f53>] SyS_mount+0x83/0xc0 Nov 23 17:41:22 ip-10-0-1-123 kernel: [606120.878662] [<ffffffff8173a9dd>] system_call_fastpath+0x1a/0x1f
如果要使用环回设备装入多设备文件系统，使用mount -o循环是不够的。 相反，你将不得不手动设置环回：