| Brendan Gregg | 9c1f362 | 2016-02-14 23:14:12 -0800 | [diff] [blame] | 1 | Demonstrations of zfsdist, the Linux eBPF/bcc version. |
| 2 | |
| 3 | |
| 4 | zfsdist traces ZFS reads, writes, opens, and fsyncs, and summarizes their |
| 5 | latency as a power-of-2 histogram. It has been written to work on ZFS on Linux |
| 6 | (http://zfsonlinux.org). For example: |
| 7 | |
| 8 | # ./zfsdist |
| 9 | Tracing ZFS operation latency... Hit Ctrl-C to end. |
| 10 | ^C |
| 11 | |
| 12 | operation = 'read' |
| 13 | usecs : count distribution |
| 14 | 0 -> 1 : 0 | | |
| 15 | 2 -> 3 : 0 | | |
| 16 | 4 -> 7 : 4479 |****************************************| |
| 17 | 8 -> 15 : 1028 |********* | |
| 18 | 16 -> 31 : 14 | | |
| 19 | 32 -> 63 : 1 | | |
| 20 | 64 -> 127 : 2 | | |
| 21 | 128 -> 255 : 6 | | |
| 22 | 256 -> 511 : 1 | | |
| 23 | 512 -> 1023 : 1256 |*********** | |
| 24 | 1024 -> 2047 : 9 | | |
| 25 | 2048 -> 4095 : 1 | | |
| 26 | 4096 -> 8191 : 2 | | |
| 27 | |
| 28 | operation = 'write' |
| 29 | usecs : count distribution |
| 30 | 0 -> 1 : 0 | | |
| 31 | 2 -> 3 : 0 | | |
| 32 | 4 -> 7 : 0 | | |
| 33 | 8 -> 15 : 0 | | |
| 34 | 16 -> 31 : 0 | | |
| 35 | 32 -> 63 : 0 | | |
| 36 | 64 -> 127 : 0 | | |
| 37 | 128 -> 255 : 75 |****************************************| |
| 38 | 256 -> 511 : 11 |***** | |
| 39 | 512 -> 1023 : 0 | | |
| 40 | 1024 -> 2047 : 0 | | |
| 41 | 2048 -> 4095 : 0 | | |
| 42 | 4096 -> 8191 : 0 | | |
| 43 | 8192 -> 16383 : 0 | | |
| 44 | 16384 -> 32767 : 0 | | |
| 45 | 32768 -> 65535 : 0 | | |
| 46 | 65536 -> 131071 : 13 |****** | |
| 47 | 131072 -> 262143 : 1 | | |
| 48 | |
| 49 | operation = 'open' |
| 50 | usecs : count distribution |
| 51 | 0 -> 1 : 0 | | |
| 52 | 2 -> 3 : 2 |****************************************| |
| 53 | |
| 54 | This output shows a bimodal distribution for read latency, with a faster |
| 55 | mode of around 5 thousand reads that took between 4 and 15 microseconds, and a |
| 56 | slower mode of 1256 reads that took between 512 and 1023 microseconds. It's |
| 57 | likely that the faster mode was a hit from the in-memory file system cache, |
| 58 | and the slower mode is a read from a storage device (disk). |
| 59 | |
| 60 | The write latency is also bimodal, with a faster mode between 128 and 511 us, |
| 61 | and the slower mode between 65 and 131 ms. |
| 62 | |
| 63 | This "latency" is measured from when the operation was issued from the VFS |
| 64 | interface to the file system (via the ZFS POSIX layer), to when it completed. |
| 65 | This spans everything: block device I/O (disk I/O), file system CPU cycles, |
| 66 | file system locks, run queue latency, etc. This is a better measure of the |
| 67 | latency suffered by applications reading from the file system than measuring |
| 68 | this down at the block device interface. |
| 69 | |
| 70 | Note that this only traces the common file system operations previously |
| 71 | listed: other file system operations (eg, inode operations including |
| 72 | getattr()) are not traced. |
| 73 | |
| 74 | |
| 75 | An optional interval and a count can be provided, as well as -m to show the |
| 76 | distributions in milliseconds. For example: |
| 77 | |
| 78 | # ./zfsdist 1 5 |
| 79 | Tracing ZFS operation latency... Hit Ctrl-C to end. |
| 80 | |
| 81 | 06:55:41: |
| 82 | |
| 83 | operation = 'read' |
| 84 | usecs : count distribution |
| 85 | 0 -> 1 : 0 | | |
| 86 | 2 -> 3 : 0 | | |
| 87 | 4 -> 7 : 3976 |****************************************| |
| 88 | 8 -> 15 : 1181 |*********** | |
| 89 | 16 -> 31 : 18 | | |
| 90 | 32 -> 63 : 4 | | |
| 91 | 64 -> 127 : 17 | | |
| 92 | 128 -> 255 : 16 | | |
| 93 | 256 -> 511 : 0 | | |
| 94 | 512 -> 1023 : 1275 |************ | |
| 95 | 1024 -> 2047 : 36 | | |
| 96 | 2048 -> 4095 : 3 | | |
| 97 | 4096 -> 8191 : 0 | | |
| 98 | 8192 -> 16383 : 1 | | |
| 99 | 16384 -> 32767 : 1 | | |
| 100 | |
| 101 | 06:55:42: |
| 102 | |
| 103 | operation = 'read' |
| 104 | usecs : count distribution |
| 105 | 0 -> 1 : 0 | | |
| 106 | 2 -> 3 : 0 | | |
| 107 | 4 -> 7 : 12751 |****************************************| |
| 108 | 8 -> 15 : 1190 |*** | |
| 109 | 16 -> 31 : 38 | | |
| 110 | 32 -> 63 : 7 | | |
| 111 | 64 -> 127 : 85 | | |
| 112 | 128 -> 255 : 47 | | |
| 113 | 256 -> 511 : 0 | | |
| 114 | 512 -> 1023 : 1010 |*** | |
| 115 | 1024 -> 2047 : 49 | | |
| 116 | 2048 -> 4095 : 12 | | |
| 117 | |
| 118 | 06:55:43: |
| 119 | |
| 120 | operation = 'read' |
| 121 | usecs : count distribution |
| 122 | 0 -> 1 : 0 | | |
| 123 | 2 -> 3 : 0 | | |
| 124 | 4 -> 7 : 80925 |****************************************| |
| 125 | 8 -> 15 : 1645 | | |
| 126 | 16 -> 31 : 251 | | |
| 127 | 32 -> 63 : 24 | | |
| 128 | 64 -> 127 : 16 | | |
| 129 | 128 -> 255 : 12 | | |
| 130 | 256 -> 511 : 0 | | |
| 131 | 512 -> 1023 : 80 | | |
| 132 | 1024 -> 2047 : 1 | | |
| 133 | |
| 134 | 06:55:44: |
| 135 | |
| 136 | operation = 'read' |
| 137 | usecs : count distribution |
| 138 | 0 -> 1 : 0 | | |
| 139 | 2 -> 3 : 0 | | |
| 140 | 4 -> 7 : 81207 |****************************************| |
| 141 | 8 -> 15 : 2075 |* | |
| 142 | 16 -> 31 : 2005 | | |
| 143 | 32 -> 63 : 177 | | |
| 144 | 64 -> 127 : 3 | | |
| 145 | |
| 146 | 06:55:45: |
| 147 | |
| 148 | operation = 'read' |
| 149 | usecs : count distribution |
| 150 | 0 -> 1 : 0 | | |
| 151 | 2 -> 3 : 0 | | |
| 152 | 4 -> 7 : 74364 |****************************************| |
| 153 | 8 -> 15 : 865 | | |
| 154 | 16 -> 31 : 4960 |** | |
| 155 | 32 -> 63 : 625 | | |
| 156 | 64 -> 127 : 2 | | |
| 157 | |
| 158 | This workload was randomly reading from a file that became cached. The slower |
| 159 | mode can be seen to disappear by the final summaries. |
| 160 | |
| 161 | |
| 162 | USAGE message: |
| 163 | |
| 164 | # ./zfsdist -h |
| 165 | usage: zfsdist [-h] [-T] [-m] [-p PID] [interval] [count] |
| 166 | |
| 167 | Summarize ZFS operation latency |
| 168 | |
| 169 | positional arguments: |
| 170 | interval output interval, in seconds |
| 171 | count number of outputs |
| 172 | |
| 173 | optional arguments: |
| 174 | -h, --help show this help message and exit |
| 175 | -T, --notimestamp don't include timestamp on interval output |
| 176 | -m, --milliseconds output in milliseconds |
| 177 | -p PID, --pid PID trace this PID only |
| 178 | |
| 179 | examples: |
| 180 | ./zfsdist # show operation latency as a histogram |
| 181 | ./zfsdist -p 181 # trace PID 181 only |
| 182 | ./zfsdist 1 10 # print 1 second summaries, 10 times |
| 183 | ./zfsdist -m 5 # 5s summaries, milliseconds |