UPDATE: I'm no longer having this problem on 4.9.* Not sure when it was fixed.Everyday after a full system backup, various programs are failing with read errors until I run echo 2 > /proc/sys/vm/drop_caches to free reclaimable slab objects.For example, here's the output of sudo apt-get update after a backup.$ sudo apt-get updateHit http://ftp.ca.debian.org unstable InReleaseHit http://ftp.ca.debian.org experimental InRelease                                                            Ign http://dl.google.com stable InRelease                                                                      Get:1 http://ftp.ca.debian.org unstable/contrib amd64 Packages/DiffIndex [7,819 B]               Hit http://dl.google.com stable Release.gpg                                     Hit http://ppa.launchpad.net wily InRelease                               Get:2 http://ftp.ca.debian.org unstable/non-free amd64 Packages/DiffIndex [6,577 B]            Hit http://dl.google.com stable Release                                         Hit http://ppa.launchpad.net wily InRelease                                                      Get:3 http://ftp.ca.debian.org unstable/main amd64 Packages/DiffIndex [7,876 B]Get:4 http://ftp.ca.debian.org unstable/contrib i386 Packages/DiffIndex [7,819 B]Get:5 http://ppa.launchpad.net wily/main amd64 Packages [4,559 B]Get:6 http://ftp.ca.debian.org unstable/non-free i386 Packages/DiffIndex [6,715 B]           Get:7 http://ppa.launchpad.net wily/main i386 Packages [4,608 B]                                       Get:8 http://ftp.ca.debian.org unstable/main i386 Packages/DiffIndex [7,876 B]                                 Hit http://dl.google.com stable/main amd64 Packages                                             Get:9 http://ftp.ca.debian.org unstable/contrib Translation-en/DiffIndex [2,161 B]             Get:10 http://ppa.launchpad.net wily/main Translation-en [1,663 B]Hit http://dl.google.com stable/main i386 PackagesE: Read error - read (5: Input/output error)    Another sample error, this time with gulp/node.js$ gulp watchfs.js:651  var r = binding.read(fd, buffer, offset, length, position);                  ^Error: EIO: i/o error, read    at Error (native)    at Object.fs.readSync (fs.js:651:19)    at Object.fs.readFileSync (fs.js:467:24)    at Object.Module._extensions..js (module.js:431:20)    at Module.load (module.js:356:32)    at Function.Module._load (module.js:313:12)    at Module.require (module.js:366:17)    at require (module.js:385:17)    at Object.<anonymous> (/usr/local/lib/node_modules/gulp/node_modules/liftoff/node_modules/findup-sync/lib/findup-sync.js:15:12)    at Module._compile (module.js:425:26)Other programs fail with read errors as well, it's not just apt-get and gulp/node.js.slabtop output:$ sudo slabtop -o Active / Total Objects (% used)    : 7244650 / 7322302 (98.9%) Active / Total Slabs (% used)      : 882626 / 882697 (100.0%) Active / Total Caches (% used)     : 78 / 122 (63.9%) Active / Total Size (% used)       : 3423174.16K / 3434416.86K (99.7%) Minimum / Average / Maximum Object : 0.02K / 0.47K / 4096.00K  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME                   2419584 2418888  99%    0.97K 604896        4   2419584K btrfs_inode            2249163 2249125  99%    0.19K 107103       21    428412K dentry                 1271127 1270067  99%    0.30K  97779       13    391116K btrfs_delayed_node     306243 299649  97%    0.06K   4861       63     19444K kmalloc-64             241556 230494  95%    0.27K  17254       14     69016K btrfs_extent_buffer    215068 212777  98%    0.14K   7681       28     30724K btrfs_path             186102 185989  99%    0.56K  26586        7    106344K radix_tree_node        174650 144422  82%    0.08K   3493       50     13972K btrfs_extent_state      37170  34869  93%    0.06K    590       63      2360K btrfs_free_space        37149  33473  90%    0.19K   1769       21      7076K kmalloc-192             32891  32382  98%    0.12K   1061       31      4244K kmalloc-96              26536  19327  72%    0.03K    214      124       856K kmalloc-32              24123  24015  99%    0.12K    731       33      2924K kernfs_node_cache       19656  19631  99%    0.07K    351       56      1404K Acpi-ParseExt           13728  11523  83%    0.25K    858       16      3432K kmalloc-256             11648  10783  92%    0.55K   1664        7      6656K inode_cache             11160   7283  65%    0.12K    360       31      1440K kmalloc-node            10696   9398  87%    0.07K    191       56       764K anon_vma                 7059   6714  95%    0.10K    181       39       724K blkdev_ioc               3735   3615  96%    0.05K     45       83       180K ftrace_event_field       3696   3574  96%    0.50K    462        8      1848K kmalloc-512              3018   2871  95%    0.60K    503        6      2012K proc_inode_cache         1584   1503  94%    0.04K     16       99        64K Acpi-Namespace           1464   1418  96%    0.63K    244        6       976K shmem_inode_cache        1426   1348  94%    0.09K     31       46       124K trace_event_file         1400   1382  98%    1.00K    350        4      1400K kmalloc-1024             1311   1248  95%    4.00K   1311        1      5244K kmalloc-4096             1074    985  91%    0.62K    179        6       716K sock_inode_cache          852    806  94%    0.88K    213        4       852K RAW                       726    718  98%    2.94K    363        2      2904K task_struct               612    608  99%    2.00K    306        2      1224K kmalloc-2048              462    447  96%    2.05K    154        3      1232K idr_layer_cache           462    210  45%    0.18K     21       22        84K btrfs_trans_handle        429    157  36%    0.10K     11       39        44K buffer_head               384    181  47%    0.31K     32       12       128K bio-1                     355    217  61%    0.05K      5       71        20K file_lock_ctx             350    307  87%    1.12K     50        7       400K signal_cache              327    307  93%    2.06K    109        3       872K sighand_cache             289    211  73%    0.23K     17       17        68K cfq_queue                 280    156  55%    0.38K     28       10       112K mnt_cache              free output:$ sudo free -h              total        used        free      shared  buff/cache   availableMem:            15G        2.3G        292M         28M         13G         13GSwap:          7.5G        4.9M        7.4GAfter running echo 2 > /proc/sys/vm/drop_caches the errors no longer appear. apt-get and other programs run fine.slabtop output:$ sudo slabtop -o Active / Total Objects (% used)    : 586239 / 777567 (75.4%) Active / Total Slabs (% used)      : 57059 / 57123 (99.9%) Active / Total Caches (% used)     : 79 / 122 (64.8%) Active / Total Size (% used)       : 180630.05K / 229256.91K (78.8%) Minimum / Average / Maximum Object : 0.02K / 0.29K / 4096.00K  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME                   241556 230586  95%    0.27K  17254       14     69016K btrfs_extent_buffer    146251 100390  68%    0.56K  20893        7     83572K radix_tree_node         50967  26035  51%    0.06K    809       63      3236K kmalloc-64              37170  34866  93%    0.06K    590       63      2360K btrfs_free_space        37149  33440  90%    0.19K   1769       21      7076K kmalloc-192             37016   7054  19%    0.14K   1322       28      5288K btrfs_path              35889  13681  38%    0.19K   1709       21      6836K dentry                  27700   1805   6%    0.08K    554       50      2216K btrfs_extent_state      26412  19384  73%    0.03K    213      124       852K kmalloc-32              24123  24067  99%    0.12K    731       33      2924K kernfs_node_cache       19656  19637  99%    0.07K    351       56      1404K Acpi-ParseExt           13712  11542  84%    0.25K    857       16      3428K kmalloc-256             12152   8791  72%    0.97K   3038        4     12152K btrfs_inode             10696   9414  88%    0.07K    191       56       764K anon_vma                 9632   8948  92%    0.55K   1376        7      5504K inode_cache              8742   4845  55%    0.12K    282       31      1128K kmalloc-node             7059   6794  96%    0.10K    181       39       724K blkdev_ioc               4867   2606  53%    0.12K    157       31       628K kmalloc-96               3735   3710  99%    0.05K     45       83       180K ftrace_event_field       3688   3525  95%    0.50K    461        8      1844K kmalloc-512              1794    498  27%    0.30K    138       13       552K btrfs_delayed_node       1584   1521  96%    0.04K     16       99        64K Acpi-Namespace           1464   1418  96%    0.63K    244        6       976K shmem_inode_cache        1426   1348  94%    0.09K     31       46       124K trace_event_file         1420   1357  95%    1.00K    355        4      1420K kmalloc-1024             1310   1252  95%    4.00K   1310        1      5240K kmalloc-4096             1074   1016  94%    0.62K    179        6       716K sock_inode_cache          852    807  94%    0.88K    213        4       852K RAW                       726    713  98%    2.94K    363        2      2904K task_struct               648    254  39%    0.60K    108        6       432K proc_inode_cache          636    635  99%    2.00K    318        2      1272K kmalloc-2048              506    240  47%    0.18K     23       22        92K btrfs_trans_handle        468    190  40%    0.10K     12       39        48K buffer_head               462    447  96%    2.05K    154        3      1232K idr_layer_cache           408    250  61%    0.31K     34       12       136K bio-1                     355    161  45%    0.05K      5       71        20K file_lock_ctx             350    307  87%    1.12K     50        7       400K signal_cache              327    307  93%    2.06K    109        3       872K sighand_cache             300    286  95%    0.38K     30       10       120K mnt_cache                 297     85  28%    0.04K      3       99        12K btrfs_delayed_extent_opfree output:$ sudo free -h              total        used        free      shared  buff/cache   availableMem:            15G        2.3G        6.1G         28M        7.3G         13GSwap:          7.5G        4.8M        7.4GI'm running Debian Sid on a btrfs but I've had the same problem with ext4 so I don't think it's a filesystem specific issue.$ uname -v#1 SMP Debian 4.2.6-1 (2015-11-10)I've also tried to set vfs_cache_pressure to a high value.vm.vfs_cache_pressure=[ 100 | 1000 | 100000 ]I get less read errors at 100000 but CPU usage climbs and system responsiveness becomes terrible so I'm looking for an alternate solution.I could stick echo 2 > /proc/sys/vm/drop_caches into a cron job but that's not fixing the problem, it's just workaround.Here's dmesg
Why are slab objects not reclaimed automatically
linux kernel;memory;cache