BUG: dir_*_usage stats are reported as 0 for read-only bookies after a restart

[pasha-kuznetsov]

When a read-only bookie is restarted it keeps reporting dir_*_usage stats as 0 until it becomes writable again.

This is caused by the LedgerDirsMonitor.check only updating diskUsages if there are any writable dirs, or if the total usage goes below the low water mark, otherwise relying on previously filled values which are 0 after a bookie is restarted.

Source:

    private void check() {
        try {
            List<File> writableDirs = ldm.getWritableLedgerDirs();
            // Check all writable dirs disk space usage.
            for (File dir : writableDirs) {
                try {
                    diskUsages.put(dir, diskChecker.checkDir(dir));
                    ...
            }
        ...
        List<File> fullfilledDirs = new ArrayList<File>(ldm.getFullFilledLedgerDirs());
        boolean hasWritableLedgerDirs = ldm.hasWritableLedgerDirs();
        float totalDiskUsage = 0;

        // When bookie is in READONLY mode .i.e there are no writableLedgerDirs:
        // - Check if the total disk usage is below DiskLowWaterMarkUsageThreshold.
        // - If So, walk through the entire list of fullfilledDirs and add them back to writableLedgerDirs list if
        // their usage is < conf.getDiskUsageThreshold.
        try {
            if (hasWritableLedgerDirs
                    || (totalDiskUsage = diskChecker.getTotalDiskUsage(ldm.getAllLedgerDirs())) < conf
                            .getDiskLowWaterMarkUsageThreshold()) {
                // Check all full-filled disk space usage
                for (File dir : fullfilledDirs) {
                    try {
                        diskUsages.put(dir, diskChecker.checkDir(dir));
                        ...