Hello,
so I have an embedded application with limited flash write cycles. I am writing 2 blocks to disk 5 times a second (ext4, block size 1024).
The data written is binary and fixed size and to date I open the files with (O_CREAT | O_WRONLY | O_DSYNC | O_TRUNC).
I set out to measure whether this O_TRUNC leads to an extra write (because I might be able to do without - data is fixed size) and understand the linux tooling around for inspection.
I wrote a test script, that creates a 2MB ext4 file system on a separate block device, so I can trace with blktrace -d /dev/sdc
If somebody please could have a look if the interpretation of the traces is correct and what the missing identifiers are.
Below is the output from blkparse for the moments where I write with open (DSYNC | TRUNC)
>
> 8,32 2 152 8.831293895 22900 C RAM 2060 + 2 [0] <-- previous _C_omplete
> 8,32 10 2 9.245982628 14449 D N 0 [kworker/u40:2] <-- D =issued
> 8,32 2 153 9.246369033 22900 C N [0] <-- C_omplete
> 8,32 0 200 9.268019255 15706 A RM 2062 + 2 <- (8,33) 14 <-- A = remap
> 8,32 0 201 9.268020922 15706 Q RM 2062 + 2 [writerdt] <-- Queued
> 8,32 0 202 9.268034745 15706 G RM 2062 + 2 [writerdt] <-- Get = allocate Req.
> 8,32 0 203 9.268051167 15706 I RM 2062 + 2 [writerdt] <-- I_nserted in Queue
> 8,32 0 204 9.268107127 161 D RM 2062 + 2 [kworker/0:1H] <-- D issued "R"ead
> 8,32 2 154 9.268704050 22900 C RM 2062 + 2 [0]
> 8,32 0 205 9.268881750 15706 A WS 2160 + 2 <- (8,33) 112
> 8,32 0 206 9.268882510 15706 Q WS 2160 + 2 [writerdt]
> 8,32 0 207 9.268890935 15706 G WS 2160 + 2 [writerdt]
> 8,32 0 208 9.268891685 15706 P N [writerdt]
> 8,32 0 209 9.268892510 15706 U N [writerdt] 1
> 8,32 0 210 9.268895672 15706 I WS 2160 + 2 [writerdt]
> 8,32 0 211 9.268913234 15706 D WS 2160 + 2 [writerdt] <-- D issued "W"rite "S"ynchronous
> 8,32 2 155 9.271009859 22900 C WS 2160 + 2 [0]
> 8,32 0 212 9.271041534 15706 A WSM 2126 + 2 <- (8,33) 78
> 8,32 0 213 9.271041910 15706 Q WSM 2126 + 2 [writerdt]
> 8,32 0 214 9.271051615 15706 G WSM 2126 + 2 [writerdt]
> 8,32 0 215 9.271053774 15706 I WSM 2126 + 2 [writerdt]
> 8,32 0 216 9.271075678 161 D WSM 2126 + 2 [kworker/0:1H] <-- D issued "W"rite "S"ynchronous
> 8,32 2 156 9.273122709 22900 C WSM 2126 + 2 [0]
> 8,32 4 67 10.277429577 13420 A WM 2050 + 2 <- (8,33) 2
-> What are M and N in the "rwbs" field? I could not find this in the manpage.
-> So it looks like this does 2 write operations. This is consistent with ext4 changing the extent for the file (for truncing to 0-length first).
Now I do this again without truncation and get
>
> 8,32 8 12 12.446019890 15060 D N 0 [kworker/u40:10]
> 8,32 0 223 12.446327254 22900 C N [0]
> 8,32 12 18 14.462068679 15060 D N 0 [kworker/u40:10]
> 8,32 0 224 14.462286092 22900 C N [0]
> 8,32 1 4 14.471326135 15725 A RM 2062 + 2 <- (8,33) 14
> 8,32 1 5 14.471327925 15725 Q RM 2062 + 2 [writerd]
> 8,32 1 6 14.471344608 15725 G RM 2062 + 2 [writerd]
> 8,32 1 7 14.471361723 15725 I RM 2062 + 2 [writerd]
> 8,32 1 8 14.471421391 358 D RM 2062 + 2 [kworker/1:1H] <-- D issued "R"ead
> 8,32 0 225 14.472161215 22900 C RM 2062 + 2 [0]
> 8,32 1 9 14.472240211 15725 A R 2158 + 2 <- (8,33) 110
> 8,32 1 10 14.472240898 15725 Q R 2158 + 2 [writerd]
> 8,32 1 11 14.472247182 15725 G R 2158 + 2 [writerd]
> 8,32 1 12 14.472250123 15725 I R 2158 + 2 [writerd]
> 8,32 1 13 14.472273237 358 D R 2158 + 2 [kworker/1:1H] <-- D issued "R"ead
> 8,32 0 226 14.472751621 22900 C R 2158 + 2 [0]
> 8,32 1 14 14.472792434 15725 A WS 2158 + 2 <- (8,33) 110
> 8,32 1 15 14.472792995 15725 Q WS 2158 + 2 [writerd]
> 8,32 1 16 14.472798733 15725 G WS 2158 + 2 [writerd]
> 8,32 1 17 14.472799196 15725 P N [writerd]
> 8,32 1 18 14.472800161 15725 U N [writerd] 1
> 8,32 1 19 14.472802573 15725 I WS 2158 + 2 [writerd]
> 8,32 1 20 14.472812600 15725 D WS 2158 + 2 [writerd] <-- D issued "W"rite "S"ynchronous
> 8,32 0 227 14.474721702 22900 C WS 2158 + 2 [0]
> 8,32 9 84 14.621987608 15718 A RAM 2060 + 2 <- (8,33) 12
> 8,32 9 85 14.621989072 15718 Q RAM 2060 + 2 [ext4lazyinit]
> 8,32 9 86 14.622005410 15718 G RAM 2060 + 2 [ext4lazyinit]
> 8,32 9 87 14.622006544 15718 P N [ext4lazyinit]
> 8,32 9 88 14.622007637 15718 U N [ext4lazyinit] 1
> 8,32 9 89 14.622023937 15718 I RAM 2060 + 2 [ext4lazyinit]
> 8,32 9 90 14.622060975 15718 D RAM 2060 + 2 [ext4lazyinit]
> 8,32 0 228 14.622902229 22900 C RAM 2060 + 2 [0]
> 8,32 5 29 15.479070846 14449 A WM 2050 + 2 <- (8,33) 2
-> and only get one issued Write (rewriting one block), but 2 reads.
This seems very consistend with the statistics, however, why is only 1/2 writes "dispatched"? Do reads never get dispatched or is it just not accounted in the process because of kernel worker?
> blkparse -s reports for the writing processes
>
> writerdt (15706)
> Reads Queued: 1, 1KiB Writes Queued: 2, 2KiB
> Read Dispatches: 0, 0KiB Write Dispatches: 1, 1KiB
> Reads Requeued: 0 Writes Requeued: 0
> Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
> Read Merges: 0, 0KiB Write Merges: 0, 0KiB
> IO unplugs: 1 Timer unplugs: 0
>
> writerd (15725)
> Reads Queued: 2, 2KiB Writes Queued: 1, 1KiB
> Read Dispatches: 0, 0KiB Write Dispatches: 1, 1KiB
> Reads Requeued: 0 Writes Requeued: 0
> Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
> Read Merges: 0, 0KiB Write Merges: 0, 0KiB
> IO unplugs: 1 Timer unplugs: 0
>
My conclusion for now:
If your data is fixed-length, and you are guaranteed to rewrite the file contents in full, do not do O_TRUNC, because it saves one write to the block device (on ext4) - but on the cost of one extra read.
Thanks,
Max
Addendum: debugfs inspections show that writing with O_TRUNC changes the extent on the file, but withouth O_TRUNC it doesn't
> -- before test
>
> debugfs: stat t_dsync_trunc.txt
> Inode: 12 Generation: 2004264324 Version: 0x00000001
> EXTENTS: (0):54
>
> debugfs: stat t_dsync_notrunc.txt
> Inode: 13 Generation: 1582093351 Version: 0x00000001
> EXTENTS: (0):55
>
>
> -- after writing to t_dsync_trunc.txt
>
> debugfs: stat t_dsync_trunc.txt
> Inode: 12 Generation: 2004264324 Version: 0x00000002
> EXTENTS: (0):56
>
> debugfs: stat t_dsync_notrunc.txt
> Inode: 13 Generation: 1582093351 Version: 0x00000001
> EXTENTS: (0):55
>
>
> -- after writing to t_dsync_notrunc.txt
>
> debugfs: stat t_dsync_trunc.txt
> Inode: 12 Generation: 2004264324 Version: 0x00000002
> EXTENTS: (0):56
>
> debugfs: stat t_dsync_notrunc.txt
> Inode: 13 Generation: 1582093351 Version: 0x00000002
> EXTENTS: (0):55
Addendum: trace-cmd for writerdt
> ext4lazyinit-15688 [005] 59288.685923: ext4_prefetch_bitmaps: dev 8,33 group 0 next 0 ios 1
> writerdt-15706 [000] 59289.122525: ext4_es_lookup_extent_enter: dev 8,33 ino 2 lblk 0
> writerdt-15706 [000] 59289.122527: ext4_es_lookup_extent_exit: dev 8,33 ino 2 found 0 [0/0) 0
> writerdt-15706 [000] 59289.122529: ext4_ext_map_blocks_enter: dev 8,33 ino 2 lblk 0 len 1 flags
> writerdt-15706 [000] 59289.122531: ext4_es_cache_extent: dev 8,33 ino 2 es [0/1) mapped 7 status W
> writerdt-15706 [000] 59289.122533: ext4_ext_show_extent: dev 8,33 ino 2 lblk 0 pblk 7 len 1
> writerdt-15706 [000] 59289.122534: ext4_ext_map_blocks_exit: dev 8,33 ino 2 flags lblk 0 pblk 7 len 1 mflags M ret 1
> writerdt-15706 [000] 59289.122536: ext4_es_insert_extent: dev 8,33 ino 2 es [0/1) mapped 7 status W
> writerdt-15706 [000] 59289.123269: ext4_journal_start: dev 8,33 blocks 1, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b4f38a5S
> writerdt-15706 [000] 59289.123285: ext4_journal_start: dev 8,33 blocks 3, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50e40aS
> writerdt-15706 [000] 59289.123287: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b50e814S
> writerdt-15706 [000] 59289.123292: ext4_truncate_enter: dev 8,33 ino 12 blocks 2
> writerdt-15706 [000] 59289.123294: ext4_journal_start: dev 8,33 blocks 6, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50beafS
> writerdt-15706 [000] 59289.123295: ext4_discard_preallocations: dev 8,33 ino 12 len: 0 needed 0
> writerdt-15706 [000] 59289.123296: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b4ee524S
> writerdt-15706 [000] 59289.123297: ext4_es_remove_extent: dev 8,33 ino 12 es [0/4294967295)
> writerdt-15706 [000] 59289.123298: ext4_journal_start: dev 8,33 blocks 1, rsv_blocks 0, revoke_creds 0, caller 0xffffffff8b4ecc8fS
> writerdt-15706 [000] 59289.123299: ext4_ext_remove_space: dev 8,33 ino 12 since 0 end 4294967294 depth 0
> writerdt-15706 [000] 59289.123300: ext4_ext_rm_leaf: dev 8,33 ino 12 start_lblk 0 last_extent [0(54), 1]partial [pclu 0 lblk 0 state 0]
> writerdt-15706 [000] 59289.123302: ext4_remove_blocks: dev 8,33 ino 12 extent [0(54), 1]from 0 to 0 partial [pclu 0 lblk 0 state 0]
> writerdt-15706 [000] 59289.123303: ext4_free_blocks: dev 8,33 ino 12 mode 0>o< block 33188 count 54 flags 1
> writerdt-15706 [000] 59289.123306: ext4_mballoc_free: dev 8,33 inode 12 extent 0/53/1
> writerdt-15706 [000] 59289.123313: ext4_journal_start: dev 8,33 blocks 2, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50e969S
> writerdt-15706 [000] 59289.123313: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b50e98bS
> writerdt-15706 [000] 59289.123317: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b4e6ca4S
> writerdt-15706 [000] 59289.123318: ext4_ext_remove_space_done: dev 8,33 ino 12 since 0 end 4294967294 depth 0 partial [pclu 0 lblk 0 state 0] remaining_entries 0
> writerdt-15706 [000] 59289.123319: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b4e6ca4S
> writerdt-15706 [000] 59289.123320: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b50bf86S
> writerdt-15706 [000] 59289.123321: ext4_truncate_exit: dev 8,33 ino 12 blocks 0
> writerdt-15706 [000] 59289.123322: ext4_journal_start: dev 8,33 blocks 2, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50e969S
> writerdt-15706 [000] 59289.123322: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b50e98bS
> writerdt-15706 [000] 59289.123334: ext4_write_begin: dev 8,33 ino 12 pos 0 len 128
> writerdt-15706 [000] 59289.123341: ext4_journal_start: dev 8,33 blocks 7, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50c313S
> writerdt-15706 [000] 59289.123342: ext4_es_lookup_extent_enter: dev 8,33 ino 12 lblk 0
> writerdt-15706 [000] 59289.123343: ext4_es_lookup_extent_exit: dev 8,33 ino 12 found 0 [0/0) 0
> writerdt-15706 [000] 59289.123343: ext4_ext_map_blocks_enter: dev 8,33 ino 12 lblk 0 len 1 flags
> writerdt-15706 [000] 59289.123345: ext4_es_find_extent_range_enter: dev 8,33 ino 12 lblk 0
> writerdt-15706 [000] 59289.123346: ext4_es_find_extent_range_exit: dev 8,33 ino 12 es [0/0) mapped 0 status
> writerdt-15706 [000] 59289.123346: ext4_es_insert_extent: dev 8,33 ino 12 es [0/4294967295) mapped 0 status H
> writerdt-15706 [000] 59289.123347: ext4_ext_map_blocks_exit: dev 8,33 ino 12 flags lblk 0 pblk 0 len 1 mflags ret 0
> writerdt-15706 [000] 59289.123348: ext4_ext_map_blocks_enter: dev 8,33 ino 12 lblk 0 len 1 flags CREATE
> writerdt-15706 [000] 59289.123349: ext4_request_blocks: dev 8,33 ino 12 flags HINT_DATA len 1 lblk 0 goal 8193 lleft 0 lright 0 pleft 0 pright 0
> writerdt-15706 [000] 59289.123353: ext4_journal_start: dev 8,33 blocks 2, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50e969S
> writerdt-15706 [000] 59289.123353: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b50e98bS
> writerdt-15706 [000] 59289.123372: ext4_mb_new_group_pa: dev 8,33 ino 12 pstart 56 len 512 lstart 56
> writerdt-15706 [000] 59289.123376: ext4_mballoc_alloc: dev 8,33 inode 12 orig 0/0/1@0 goal 0/0/512@0 result 0/55/512@0 blks 5 grps 1 cr 2 flags HINT_DATA|HINT_GRP_ALLOC tail 55 broken 256
> writerdt-15706 [000] 59289.123377: ext4_allocate_blocks: dev 8,33 ino 12 flags HINT_DATA len 1 block 56 lblk 0 goal 8193 lleft 0 lright 0 pleft 0 pright 0
> writerdt-15706 [000] 59289.123379: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b4e6ca4S
> writerdt-15706 [000] 59289.123380: ext4_ext_map_blocks_exit: dev 8,33 ino 12 flags CREATE lblk 0 pblk 56 len 1 mflags NM ret 1
> writerdt-15706 [000] 59289.123381: ext4_es_insert_extent: dev 8,33 ino 12 es [0/1) mapped 56 status W
> writerdt-15706 [000] 59289.123385: ext4_write_end: dev 8,33 ino 12 pos 0 len 128 copied 128
> writerdt-15706 [000] 59289.123387: ext4_mark_inode_dirty: dev 8,33 ino 12 caller 0xffffffff8b50d462S
> writerdt-15706 [000] 59289.123391: ext4_sync_file_enter: dev 8,33 ino 12 parent 2 datasync 1
> writerdt-15706 [000] 59289.123393: ext4_writepages: dev 8,33 ino 12 nr_to_write 9223372036854775807 pages_skipped 0 range_start 0 range_end 127 sync_mode 1 for_kupdate 0 range_cyclic 0 writeback_index 0
> writerdt-15706 [000] 59289.123402: ext4_journal_start: dev 8,33 blocks 6, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50a8baS
> writerdt-15706 [000] 59289.123402: ext4_da_write_pages: dev 8,33 ino 12 first_page 0 nr_to_write 9223372036854775807 sync_mode 1
> writerdt-15706 [000] 59289.123444: ext4_writepages_result: dev 8,33 ino 12 ret 0 pages_written 1 pages_skipped 0 sync_mode 1 writeback_index 0
> writerdt-15706 [000] 59289.127668: ext4_sync_file_exit: dev 8,33 ino 12 ret 0
> writerdt-15706 [000] 59289.127675: ext4_alloc_da_blocks: dev 8,33 ino 12 reserved_data_blocks 0
> writerdt-15706 [000] 59289.127683: ext4_discard_preallocations: dev 8,33 ino 12 len: 0 needed 0
> sync-15708 [005] 59290.134496: ext4_sync_fs: dev 7,5 wait 0
> sync-15708 [005] 59290.134503: ext4_sync_fs: dev 8,33 wait 0
Addendum: trac-cmd for writerd
> sync-15719 [005] 59290.312417: ext4_sync_fs: dev 8,33 wait 1
> writerd-15725 [001] 59294.325659: ext4_es_lookup_extent_enter: dev 8,33 ino 2 lblk 0
> writerd-15725 [001] 59294.325661: ext4_es_lookup_extent_exit: dev 8,33 ino 2 found 0 [0/0) 0
> writerd-15725 [001] 59294.325663: ext4_ext_map_blocks_enter: dev 8,33 ino 2 lblk 0 len 1 flags
> writerd-15725 [001] 59294.325665: ext4_es_cache_extent: dev 8,33 ino 2 es [0/1) mapped 7 status W
> writerd-15725 [001] 59294.325677: ext4_ext_show_extent: dev 8,33 ino 2 lblk 0 pblk 7 len 1
> writerd-15725 [001] 59294.325678: ext4_ext_map_blocks_exit: dev 8,33 ino 2 flags lblk 0 pblk 7 len 1 mflags M ret 1
> writerd-15725 [001] 59294.325679: ext4_es_insert_extent: dev 8,33 ino 2 es [0/1) mapped 7 status W
> writerd-15725 [001] 59294.326569: ext4_journal_start: dev 8,33 blocks 1, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b4f38a5S
> writerd-15725 [001] 59294.326589: ext4_journal_start: dev 8,33 blocks 2, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50e969S
> writerd-15725 [001] 59294.326590: ext4_mark_inode_dirty: dev 8,33 ino 13 caller 0xffffffff8b50e98bS
> writerd-15725 [001] 59294.326597: ext4_write_begin: dev 8,33 ino 13 pos 0 len 128
> writerd-15725 [001] 59294.326603: ext4_journal_start: dev 8,33 blocks 7, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50c313S
> writerd-15725 [001] 59294.326605: ext4_es_lookup_extent_enter: dev 8,33 ino 13 lblk 0
> writerd-15725 [001] 59294.326605: ext4_es_lookup_extent_exit: dev 8,33 ino 13 found 0 [0/0) 0
> writerd-15725 [001] 59294.326606: ext4_ext_map_blocks_enter: dev 8,33 ino 13 lblk 0 len 1 flags
> writerd-15725 [001] 59294.326607: ext4_es_cache_extent: dev 8,33 ino 13 es [0/1) mapped 55 status W
> writerd-15725 [001] 59294.326608: ext4_ext_show_extent: dev 8,33 ino 13 lblk 0 pblk 55 len 1
> writerd-15725 [001] 59294.326609: ext4_ext_map_blocks_exit: dev 8,33 ino 13 flags lblk 0 pblk 55 len 1 mflags M ret 1
> writerd-15725 [001] 59294.326609: ext4_es_insert_extent: dev 8,33 ino 13 es [0/1) mapped 55 status W
> writerd-15725 [001] 59294.327134: ext4_write_end: dev 8,33 ino 13 pos 0 len 128 copied 128
> writerd-15725 [001] 59294.327137: ext4_mark_inode_dirty: dev 8,33 ino 13 caller 0xffffffff8b50d462S
> writerd-15725 [001] 59294.327141: ext4_sync_file_enter: dev 8,33 ino 13 parent 2 datasync 1
> writerd-15725 [001] 59294.327144: ext4_writepages: dev 8,33 ino 13 nr_to_write 9223372036854775807 pages_skipped 0 range_start 0 range_end 127 sync_mode 1 for_kupdate 0 range_cyclic 0 writeback_index 0
> writerd-15725 [001] 59294.327156: ext4_journal_start: dev 8,33 blocks 6, rsv_blocks 0, revoke_creds 8, caller 0xffffffff8b50a8baS
> writerd-15725 [001] 59294.327156: ext4_da_write_pages: dev 8,33 ino 13 first_page 0 nr_to_write 9223372036854775807 sync_mode 1
> writerd-15725 [001] 59294.327188: ext4_writepages_result: dev 8,33 ino 13 ret 0 pages_written 1 pages_skipped 0 sync_mode 1 writeback_index 0
> writerd-15725 [001] 59294.329112: ext4_sync_file_exit: dev 8,33 ino 13 ret 0
> writerd-15725 [001] 59294.329118: ext4_discard_preallocations: dev 8,33 ino 13 len: 0 needed 0
> ext4lazyinit-15718 [009] 59294.476346: ext4_read_block_bitmap_load: dev 8,33 group 0 prefetch 1
> ext4lazyinit-15718 [009] 59294.477287: ext4_mb_bitmap_load: dev 8,33 group 0
> ext4lazyinit-15718 [009] 59294.477288: ext4_mb_buddy_bitmap_load: dev 8,33 group 0
> ext4lazyinit-15718 [009] 59294.477293: ext4_prefetch_bitmaps: dev 8,33 group 0 next 0 ios > kworker/u40:2-14449 [005] 59295.333445: ext4_writepages: dev 8,33 ino 13 nr_to_write 1024 pages_skipped 0 range_start 0 range_end 9223372036854775807 sync_mode 0 for_kupdate 0 range_cyclic 1 writeback_index 0
> kworker/u40:2-14449 [005] 59295.333447: ext4_writepages_result: dev 8,33 ino 13 ret 0 pages_written 0 pages_skipped 0 sync_mode 0 writeback_index 0
> sync-15727 [007] 59295.357066: ext4_sync_fs: dev 7,5 wait 0
> sync-15727 [007] 59295.357072: ext4_sync_fs: dev 8,33 wait 0
Addendum: writer.c sourcecode
>
> #include <unistd.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdbool.h>
>
> const char* argopt(int argc, const char *const *argv, char key){
> for(int i=1; i<argc; i++){
> const char *c = argv[i];
> if(*c!='-') continue;
> while(*++c) if(*c==key) return argv[(i+1)%argc];
> }
> return 0;
> }
>
>
> int main(int argc, char **argv){
> bool trunc = false;
> bool dsync = false;
> if(argopt(argc, argv, 't')) { trunc= true; }
> if(argopt(argc, argv, 'd')) { dsync = true;}
>
> const char *o = argopt(argc, argv, 'o');
>
> const char *c = argopt(argc, argv, 'c');
>
> if (!c) {printf("-c is empty");return 1;}
>
> int fd1;
>
> int extra = 0;
> if (trunc) extra |= O_TRUNC;
> if (dsync) extra |= O_DSYNC;
> fd1 = open(o, O_CREAT | O_WRONLY | extra, S_IRUSR | S_IWUSR);
> write(fd1,c,strlen(c));
>
> close(fd1);
>
> return 0;
> }
testrunner script
> #!/bin/bash
>
> #c860942c211b9946217df2753a0354ab0d182c0d94b5fd1cf1b9b4d74493ab9d shasumgenerate
>
> cd /dev/shm
> ln -s writer writerdt
> ln -s writer writerd
> mkdir /dev/shm/blkmnt
> umount /dev/sdc1
>
> # If enabled and the uninit_bg feature is enabled, the inode table will not be fully initialized by mke2fs. This speeds up filesystem initialization noticeably, but it requires the kernel to finish initializing the filesystem in the background when the filesystem is first mounted. If the option value is omitted, it defaults to 1 to enable lazy inode table zeroing.
> mkfs.ext4 -F -E lazy_itable_init=0,lazy_journal_init=0 /dev/sdc1 1M
> sync /dev/shm/blkmnt/
> sleep 5
> mount /dev/sdc1 /dev/shm/blkmnt
>
> echo "emptyfile" > /dev/shm/blkmnt/t_dsync_trunc.txt
> echo "emptyfile" > /dev/shm/blkmnt/t_dsync_notrunc.txt
>
> umount /dev/sdc1
> dd if=/dev/sdc1 bs=4M count=2 of=/dev/shm/emptystart.bin
>
> mount /dev/sdc1 /dev/shm/blkmnt
> sync
>
> sleep 4
> date
> DUDU=$(date +%X)"_"$(cat /proc/uptime)"_"$(uname -r)"_abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz123456789"
> time ./writerdt -d -t -o /dev/shm/blkmnt/t_dsync_trunc.txt -c "${DUDU:0:128}"
> sleep 1
> sync
>
>
> umount /dev/sdc1
> dd if=/dev/sdc1 bs=4M count=2 of=/dev/shm/after_dsync_trunc.bin
>
> mount /dev/sdc1 /dev/shm/blkmnt
> sync
>
> sleep 4
> date
> DUDU=$(date +%X)"_"$(cat /proc/uptime)"_"$(uname -r)"_abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz123456789"
> time ./writerd -d -o /dev/shm/blkmnt/t_dsync_notrunc.txt -c "${DUDU:0:128}"
> sleep 1
> sync
>
> umount /dev/sdc1
> dd if=/dev/sdc1 bs=4M count=2 of=/dev/shm/after_dsync.bin
>
> export PAGER=cat
>
> echo "emptystart"
> debugfs /dev/shm/emptystart.bin <<EOF
> stats
> stat t_dsync_trunc.txt
> stat t_dsync_notrunc.txt
> EOF
>
> echo "after dsync_trunc"
> debugfs /dev/shm/after_dsync_trunc.bin <<EOF
> stats
> stat t_dsync_trunc.txt
> stat t_dsync_notrunc.txt
> EOF
>
>
> echo "after_dsync"
> debugfs /dev/shm/after_dsync.bin <<EOF
> stats
> stat t_dsync_trunc.txt
> stat t_dsync_notrunc.txt
> EOF
