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Linux/Documentation/admin-guide/device-mapper/log-writes.rst

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Diff markup

Differences between /Documentation/admin-guide/device-mapper/log-writes.rst (Version linux-6.12-rc7) and /Documentation/admin-guide/device-mapper/log-writes.rst (Version ccs-tools-1.8.12)


  1 =============                                     
  2 dm-log-writes                                     
  3 =============                                     
  4                                                   
  5 This target takes 2 devices, one to pass all I    
  6 of the write operations to.  This is intended     
  7 to verify the integrity of metadata or data as    
  8 There is a log_write_entry written for every W    
  9 able to take arbitrary data from userspace to     
 10 that is in the WRITE requests is copied into t    
 11 exactly as it happened originally.                
 12                                                   
 13 Log Ordering                                      
 14 ============                                      
 15                                                   
 16 We log things in order of completion once we a    
 17 cache.  This means that normal WRITE requests     
 18 next REQ_PREFLUSH request.  This is to make it    
 19 the log in a way that correlates to what is on    
 20 to make it easier to detect improper waiting/f    
 21                                                   
 22 This works by attaching all WRITE requests to     
 23 Once we see a REQ_PREFLUSH request we splice t    
 24 the FLUSH request completes we log all of the     
 25 completed WRITEs, at the time the REQ_PREFLUSH    
 26 simulate the worst case scenario with regard t    
 27 following example (W means write, C means comp    
 28                                                   
 29         W1,W2,W3,C3,C2,Wflush,C1,Cflush           
 30                                                   
 31 The log would show the following:                 
 32                                                   
 33         W3,W2,flush,W1....                        
 34                                                   
 35 Again this is to simulate what is actually on     
 36 cases where a power failure at a particular po    
 37 inconsistent file system.                         
 38                                                   
 39 Any REQ_FUA requests bypass this flushing mech    
 40 they complete as those requests will obviously    
 41                                                   
 42 Any REQ_OP_DISCARD requests are treated like W    
 43 have all the DISCARD requests, and then the WR    
 44 request.  Consider the following example:         
 45                                                   
 46         WRITE block 1, DISCARD block 1, FLUSH     
 47                                                   
 48 If we logged DISCARD when it completed, the re    
 49                                                   
 50         DISCARD 1, WRITE 1, FLUSH                 
 51                                                   
 52 which isn't quite what happened and wouldn't b    
 53                                                   
 54 Target interface                                  
 55 ================                                  
 56                                                   
 57 i) Constructor                                    
 58                                                   
 59    log-writes <dev_path> <log_dev_path>           
 60                                                   
 61    ============= =============================    
 62    dev_path      Device that all of the IO wil    
 63    log_dev_path  Device where the log entries     
 64    ============= =============================    
 65                                                   
 66 ii) Status                                        
 67                                                   
 68     <#logged entries> <highest allocated secto    
 69                                                   
 70     =========================== ==============    
 71     #logged entries             Number of logg    
 72     highest allocated sector    Highest alloca    
 73     =========================== ==============    
 74                                                   
 75 iii) Messages                                     
 76                                                   
 77     mark <description>                            
 78                                                   
 79         You can use a dmsetup message to set a    
 80         For example say you want to fsck a fil    
 81         write, but first you need to replay up    
 82         we're fsck'ing something reasonable, y    
 83         this::                                    
 84                                                   
 85           mkfs.btrfs -f /dev/mapper/log           
 86           dmsetup message log 0 mark mkfs         
 87           <run test>                              
 88                                                   
 89         This would allow you to replay the log    
 90         then replay from that point on doing t    
 91         interval that you want.                   
 92                                                   
 93         Every log has a mark at the end labele    
 94                                                   
 95 Userspace component                               
 96 ===================                               
 97                                                   
 98 There is a userspace tool that will replay the    
 99 It can be found here: https://github.com/josef    
100                                                   
101 Example usage                                     
102 =============                                     
103                                                   
104 Say you want to test fsync on your file system    
105 this::                                            
106                                                   
107   TABLE="0 $(blockdev --getsz /dev/sdb) log-wr    
108   dmsetup create log --table "$TABLE"             
109   mkfs.btrfs -f /dev/mapper/log                   
110   dmsetup message log 0 mark mkfs                 
111                                                   
112   mount /dev/mapper/log /mnt/btrfs-test           
113   <some test that does fsync at the end>          
114   dmsetup message log 0 mark fsync                
115   md5sum /mnt/btrfs-test/foo                      
116   umount /mnt/btrfs-test                          
117                                                   
118   dmsetup remove log                              
119   replay-log --log /dev/sdc --replay /dev/sdb     
120   mount /dev/sdb /mnt/btrfs-test                  
121   md5sum /mnt/btrfs-test/foo                      
122   <verify md5sum's are correct>                   
123                                                   
124   Another option is to do a complicated file s    
125   system is consistent during the entire opera    
126                                                   
127   TABLE="0 $(blockdev --getsz /dev/sdb) log-wr    
128   dmsetup create log --table "$TABLE"             
129   mkfs.btrfs -f /dev/mapper/log                   
130   dmsetup message log 0 mark mkfs                 
131                                                   
132   mount /dev/mapper/log /mnt/btrfs-test           
133   <fsstress to dirty the fs>                      
134   btrfs filesystem balance /mnt/btrfs-test        
135   umount /mnt/btrfs-test                          
136   dmsetup remove log                              
137                                                   
138   replay-log --log /dev/sdc --replay /dev/sdb     
139   btrfsck /dev/sdb                                
140   replay-log --log /dev/sdc --replay /dev/sdb     
141         --fsck "btrfsck /dev/sdb" --check fua     
142                                                   
143 And that will replay the log until it sees a F    
144 and if the fsck passes it will replay to the n    
145 the fsck command exists abnormally.               
                                                      

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