Hi,
Our environment is Redhat Enterprise 6.8, upgrade to kernal 4.4.89. We use sdj(480G) as as backing devices and sdk1(20G) as cache device. To test maintenance step when hard disk fail, we plug out the backing device sdj.
Here are the results during test:
# lsscsi
-- the sdj device is gone.
# ls -l /dev/bcache4
brw-rw---- 1 root root 251, 4 Sep 30 18:00 /dev/bcache4
-- the bcache4 device is still there
# dd if=/dev/bcache4 of=/dev/null bs=1M count=100
dd: reading `/dev/bcache4': Input/output error
0+0 records in
0+0 records out
0 bytes (0 B) copied, 0.00201728 s, 0.0 kB/s
-- this is resonable
# fdisk -l /dev/bcache4
-- fdisk has no result. This is resonable
# dd if=/dev/zero of=/dev/bcache4 bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.230408 s, 455 MB/s
-- bcache4 can still be written. If the backing device is really destroyed, then application don't know that, and thought data is saved. It's not acceptable. The application should get I/O error.
# dd if=/dev/bcache4 of=/dev/null bs=1M count=100
dd: reading `/dev/bcache4': Input/output error
46+1 records in
46+1 records out
48238592 bytes (48 MB) copied, 0.120517 s, 400 MB/s
-- now read still shows error but 48MB data read out.
# fdisk -l /dev/bcache4
Note: sector size is 4096 (not 512)
Disk /dev/bcache4: 480.1 GB, 480103972864 bytes
255 heads, 63 sectors/track, 7296 cylinders
Units = cylinders of 16065 * 4096 = 65802240 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000
-- fdisk got answer. Strange. 480GB is the backing device size.
# dd if=/dev/zero of=/dev/bcache4 bs=1M
dd: writing `/dev/bcache4': No space left on device
457863+0 records in
457862+0 records out
480103972864 bytes (480 GB) copied, 620.993 s, 773 MB/s
-- keep writing to bcache4, after 480GB, it says "no space left". But the backing device is 480G, cache device is only 20G, where did the data go?
-- during dd, the /var/log/message shows error
...
Sep 30 17:52:35 s103 kernel: Buffer I/O error on dev bcache4, logical block 2224259, lost async page write
Sep 30 17:52:35 s103 kernel: Buffer I/O error on dev bcache4, logical block 2224260, lost async page write
Sep 30 17:52:35 s103 kernel: Buffer I/O error on dev bcache4, logical block 2224261, lost async page write
Sep 30 17:52:40 s103 kernel: buffer_io_error: 1093169 callbacks suppressed
Sep 30 17:52:40 s103 kernel: Buffer I/O error on dev bcache4, logical block 3317548, lost async page write
...
I think correct behaviour should be, if backing device lost, "dd" (the application) should know rightaway, got I/O error.
Appreciated for any help.
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