Apache BookKeeper中数据目录分析

    private void check(final LedgerDirsManager ldm) {
    // 对于Index, EntryLog, Journal都可以设置多个存储路径, 每一种对应一个LedgerDirsManager
    // 先获取每种对应的dirs的使用情况
        final ConcurrentMap<File, Float> diskUsages = ldm.getDiskUsages();
        try {
        //获取当前可写状态的目录
            List<File> writableDirs = ldm.getWritableLedgerDirs();
            // Check all writable dirs disk space usage.
            // 循环遍历当前可写状态的dirs的剩余可写容量，更新diskUsages
            // 同时处理各种异常，比如
            // 1. 读dir失败，回调diskFailed
            // 2. 可写容量低于警戒阈值，但还处于可写状态， 回调 diskAlmostFull
            // 3. 不可写，调用ldm.addToFilledDirs
            for (File dir : writableDirs) {
                try {
                    diskUsages.put(dir, diskChecker.checkDir(dir));
                } catch (DiskErrorException e) {
                    LOG.error("Ledger directory {} failed on disk checking : ", dir, e);
                    // Notify disk failure to all listeners
                    for (LedgerDirsListener listener : ldm.getListeners()) {
                        listener.diskFailed(dir);
                    }
                } catch (DiskWarnThresholdException e) {
                    diskUsages.compute(dir, (d, prevUsage) -> {
                        if (null == prevUsage || e.getUsage() != prevUsage) {
                            LOG.warn("Ledger directory {} is almost full : usage {}", dir, e.getUsage());
                        }
                        return e.getUsage();
                    });
                    for (LedgerDirsListener listener : ldm.getListeners()) {
                        listener.diskAlmostFull(dir);
                    }
                } catch (DiskOutOfSpaceException e) {
                    diskUsages.compute(dir, (d, prevUsage) -> {
                        if (null == prevUsage || e.getUsage() != prevUsage) {
                            LOG.error("Ledger directory {} is out-of-space : usage {}", dir, e.getUsage());
                        }
                        return e.getUsage();
                    });
                    // Notify disk full to all listeners
                    ldm.addToFilledDirs(dir);
                }
            }
            // Let's get NoWritableLedgerDirException without waiting for the next iteration
            // in case we are out of writable dirs
            // otherwise for the duration of {interval} we end up in the state where
            // bookie cannot get writable dir but considered to be writable
            // check完之前所有可写目录的最新状态后，
            // 看看现在还有没有可写的目录，没有可用的就抛出异常
            ldm.getWritableLedgerDirs();
        } catch (NoWritableLedgerDirException e) {
            LOG.warn("LedgerDirsMonitor check process: All ledger directories are non writable");
            boolean highPriorityWritesAllowed = true;
            try {
                // disk check can be frequent, so disable 'loggingNoWritable' to avoid log flooding.
                ldm.getDirsAboveUsableThresholdSize(minUsableSizeForHighPriorityWrites, false);
            } catch (NoWritableLedgerDirException e1) {
                highPriorityWritesAllowed = false;
            }
            
            //进到这里，表明没有可写的目录了，回调allDisksFull
            for (LedgerDirsListener listener : ldm.getListeners()) {
                listener.allDisksFull(highPriorityWritesAllowed);
            }
        }

        List<File> fullfilledDirs = new ArrayList<File>(ldm.getFullFilledLedgerDirs());
        boolean makeWritable = ldm.hasWritableLedgerDirs();

        // When bookie is in READONLY mode, i.e there are no writableLedgerDirs:
        // - Update fullfilledDirs disk usage.
        // - If the total disk usage is below DiskLowWaterMarkUsageThreshold
        // add fullfilledDirs back to writableLedgerDirs list if their usage is < conf.getDiskUsageThreshold.
        try {
            if (!makeWritable) {
                // 返回当前所有目录总的已用容量百分比
                float totalDiskUsage = diskChecker.getTotalDiskUsage(ldm.getAllLedgerDirs());
                if (totalDiskUsage < conf.getDiskLowWaterMarkUsageThreshold()) {
                    makeWritable = true;
                } else {
                    LOG.debug(
                        "Current TotalDiskUsage: {} is greater than LWMThreshold: {}."
                                + " So not adding any filledDir to WritableDirsList",
                        totalDiskUsage, conf.getDiskLowWaterMarkUsageThreshold());
                }
            }
            // Update all full-filled disk space usage
            // 之前处于不可写状态的目录，如果GC时清除掉一些数据，则可能变为可写状态，这里作check
            for (File dir : fullfilledDirs) {
                try {
                    diskUsages.put(dir, diskChecker.checkDir(dir));
                    if (makeWritable) {
                        ldm.addToWritableDirs(dir, true);
                    }
                } catch (DiskErrorException e) {
                    // Notify disk failure to all the listeners
                    for (LedgerDirsListener listener : ldm.getListeners()) {
                        listener.diskFailed(dir);
                    }
                } catch (DiskWarnThresholdException e) {
                    diskUsages.put(dir, e.getUsage());
                    // the full-filled dir become writable but still above the warn threshold
                    if (makeWritable) {
                        ldm.addToWritableDirs(dir, false);
                    }
                } catch (DiskOutOfSpaceException e) {
                    // the full-filled dir is still full-filled
                    diskUsages.put(dir, e.getUsage());
                }
            }
        } catch (IOException ioe) {
            LOG.error("Got IOException while monitoring Dirs", ioe);
            for (LedgerDirsListener listener : ldm.getListeners()) {
                listener.fatalError();
            }
        }
    }