Hello! When I use blkdiscard to trim a SATA SSD in its entirety, it seems that not all sectors are getting trimmed. I initially ran into this on a Sandisk SDSSDA240G, but I reproduced it on a Samsung 850 EVO M.2 250GB and a few other SATA SSDs. This happens on Fedora kernel "4.11.11-300.fc26.x86_64" and a few other Fedora kernels. For this test, I have the following SSD: [root@testbox ~]# smartctl -a /dev/sda smartctl 6.5 2016-05-07 r4318 [x86_64-linux-4.11.11-300.fc26.x86_64] (local build) Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org === START OF INFORMATION SECTION === Model Family: SandForce Driven SSDs Device Model: SanDisk SDSSDA240G Serial Number: 154307402302 LU WWN Device Id: 5 001b44 ef905163e Firmware Version: Z22000RL User Capacity: 240,057,409,536 bytes [240 GB] [...] Which at the start of the test is completely "empty": [root@testbox ~]# cat /dev/sda | od -t x1 -Ax - 000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 * 37e4896000 [root@testbox ~]# I then write a 64 bit byte offset from the start of the disk to every 64 bit word, which looks as follows: [root@testbox ~]# cat /dev/sda | od -Ax -t x1 - | head -5 000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 08 000010 00 00 00 00 00 00 00 10 00 00 00 00 00 00 00 18 000020 00 00 00 00 00 00 00 20 00 00 00 00 00 00 00 28 000030 00 00 00 00 00 00 00 30 00 00 00 00 00 00 00 38 000040 00 00 00 00 00 00 00 40 00 00 00 00 00 00 00 48 [root@testbox ~]# cat /dev/sda | od -Ax -t x1 - | head -1048576 | tail -5 ffffb0 00 00 00 00 00 ff ff b0 00 00 00 00 00 ff ff b8 ffffc0 00 00 00 00 00 ff ff c0 00 00 00 00 00 ff ff c8 ffffd0 00 00 00 00 00 ff ff d0 00 00 00 00 00 ff ff d8 ffffe0 00 00 00 00 00 ff ff e0 00 00 00 00 00 ff ff e8 fffff0 00 00 00 00 00 ff ff f0 00 00 00 00 00 ff ff f8 [root@testbox ~]# I then blkdiscard the whole SSD in one go: [root@testbox ~]# blkdiscard -v /dev/sda /dev/sda: Discarded 240057409536 bytes from the offset 0 [root@testbox ~]# >From summing iostat kB_wrtn column data during the blkdiscard, it seems that all of the sectors on the SSD should have been covered by the trim operation, but this doesn't seem to zero all of the sectors -- some of them keep their original contents: [root@testbox ~]# cat /dev/sda | od -Ax -t x1 - | head -10 000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 * 1fff000 00 00 00 00 01 ff f0 00 00 00 00 00 01 ff f0 08 1fff010 00 00 00 00 01 ff f0 10 00 00 00 00 01 ff f0 18 1fff020 00 00 00 00 01 ff f0 20 00 00 00 00 01 ff f0 28 1fff030 00 00 00 00 01 ff f0 30 00 00 00 00 01 ff f0 38 1fff040 00 00 00 00 01 ff f0 40 00 00 00 00 01 ff f0 48 1fff050 00 00 00 00 01 ff f0 50 00 00 00 00 01 ff f0 58 1fff060 00 00 00 00 01 ff f0 60 00 00 00 00 01 ff f0 68 1fff070 00 00 00 00 01 ff f0 70 00 00 00 00 01 ff f0 78 [...] More specifically, there's this pattern: [root@testbox ~]# cat /dev/sda | od -Ax -t x1 - | grep -A1 -B2 '^*' | head -20 000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 * 1fff000 00 00 00 00 01 ff f0 00 00 00 00 00 01 ff f0 08 -- 1fffff0 00 00 00 00 01 ff ff f0 00 00 00 00 01 ff ff f8 2000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 * 3fff000 00 00 00 00 03 ff f0 00 00 00 00 00 03 ff f0 08 -- 3fffff0 00 00 00 00 03 ff ff f0 00 00 00 00 03 ff ff f8 4000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 * 5fff000 00 00 00 00 05 ff f0 00 00 00 00 00 05 ff f0 08 -- 5fffff0 00 00 00 00 05 ff ff f0 00 00 00 00 05 ff ff f8 6000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 * 7fff000 00 00 00 00 07 ff f0 00 00 00 00 00 07 ff f0 08 -- 7fffff0 00 00 00 00 07 ff ff f0 00 00 00 00 07 ff ff f8 [...] Even if trimming a sector doesn't guarantee that subsequent reads will return zeroes, the weird thing is is that this behavior seems to be consistent across different SSD models, down to which specific sectors are zeroed and which are not. If I blkdiscard the whole SSD again, the same pattern remains on the disk, but if I blkdiscard it again with an appropriate offset: [root@testbox ~]# blkdiscard -v -o 1048576 /dev/sda /dev/sda: Discarded 240056360960 bytes from the offset 1048576 [root@testbox ~]# I then get an SSD that returns zeroes again from every sector: [root@testbox ~]# cat /dev/sda | od -t x1 -Ax - 000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 * 37e4896000 [root@testbox ~]# Since the pattern seems to be a consistent untrimmed 4096 bytes every 32 MiB, I wonder if there is maybe some off-by-one in discard request processing somewhere. Can anyone other than me reproduce this? Cheers, Lennert
