ZIL或称SLOG, 被用于提升ZFS系统的离散fsync性能.

类似数据库的redo log或wal.

注意

1. 每个dataset对应一个zil, 也就是说一个zpool有多个zfs的话, 如果有log设备, 那么在log设备中实际上包含了多个ZIL entry.

数据写入ZIL后(fsync), 即使服务器异常, 也可以用于恢复文件系统.

2. 并不是每一笔FSYNC都会用到ZIL, 只有小于2*zil_slog_limit 的commit操作才会用到. 如果你的zil 块设备够强的话, 可以调大伙调到UINT64_MAX,  那么就不检测了, 所有的commit都用上zil设备. 一般不需要调整

/** The zfs intent log (ZIL) saves transaction records of system calls* that change the file system in memory with enough information* to be able to replay them. These are stored in memory until* either the DMU transaction group (txg) commits them to the stable pool* and they can be discarded, or they are flushed to the stable log* (also in the pool) due to a fsync, O_DSYNC or other synchronous* requirement. In the event of a panic or power fail then those log* records (transactions) are replayed.** There is one ZIL per file system. Its on-disk (pool) format consists* of 3 parts:**      - ZIL header*      - ZIL blocks*      - ZIL records** A log record holds a system call transaction. Log blocks can* hold many log records and the blocks are chained together.* Each ZIL block contains a block pointer (blkptr_t) to the next* ZIL block in the chain. The ZIL header points to the first* block in the chain. Note there is not a fixed place in the pool* to hold blocks. They are dynamically allocated and freed as* needed from the blocks available. Figure X shows the ZIL structure:*/可调参数,
/** This global ZIL switch affects all pools*/
int zil_replay_disable = 0;    /* disable intent logging replay *//** Tunable parameter for debugging or performance analysis.  Setting* zfs_nocacheflush will cause corruption on power loss if a volatile* out-of-order write cache is enabled.*/
int zfs_nocacheflush = 0;/** Define a limited set of intent log block sizes.* These must be a multiple of 4KB. Note only the amount used (again* aligned to 4KB) actually gets written. However, we can't always just* allocate SPA_MAXBLOCKSIZE as the slog space could be exhausted.*/
uint64_t zil_block_buckets[] = {4096,               /* non TX_WRITE */8192+4096,          /* data base */32*1024 + 4096,     /* NFS writes */UINT64_MAX
};/** Use the slog as long as the current commit size is less than the* limit or the total list size is less than 2X the limit.  Limit* checking is disabled by setting zil_slog_limit to UINT64_MAX.*/
unsigned long zil_slog_limit = 1024 * 1024;
#define USE_SLOG(zilog) (((zilog)->zl_cur_used < zil_slog_limit) || \((zilog)->zl_itx_list_sz < (zil_slog_limit << 1)))#if defined(_KERNEL) && defined(HAVE_SPL)
module_param(zil_replay_disable, int, 0644);
MODULE_PARM_DESC(zil_replay_disable, "Disable intent logging replay");module_param(zfs_nocacheflush, int, 0644);
MODULE_PARM_DESC(zfs_nocacheflush, "Disable cache flushes");module_param(zil_slog_limit, ulong, 0644);
MODULE_PARM_DESC(zil_slog_limit, "Max commit bytes to separate log device");
#endif