BTRFS

De Jose Castillo Aliaga
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Aquest article no pretén substituir al manual oficial, sols és, de moment, una recopilació de apunts a partir de l'ús del sistema de fitxers.

BTRFS es compara sovint amb ZFS, ja que existeixen per les mateixes raons. La primera diferència és la forma en que han arribat a Linux i els problemes de llicències que pot donar. ZFS és més madur i,en la majoria de casos més robust. BTRFS és més flexible en algunes coses.

Avantatges experimentats per mí de BTRFS sobre ZFS:

  • Més fàcil de crear i de muntar els volums creats.
  • Si afegeixes un nou disc a un RAID1, per exemple, en ZFS sols el sincronitza en els nous arxius. En BTRFS es sincronitza si vols.
  • BTRFS funciona dins de contenedors LXD, per tant, si fem nesting podem aprofitar els avantatges dins dels subcontenidors.

Cas pràctic

En un servidor configurat com a RAID 10 amb 4 discs durs, trobem un misstage a la pantalla que diu:

BTRFS error (device sda1): bdev /dev/sdf1 errs: wr 51956436, rd 28739183, flush 124601, corrupt 0, gen 0

Això ens diu que hi ha algun problema en el sistema d'arxius. Si fem algunes comprovacions:

 sudo btrfs scrub start /media/btrfs/
 scrub started on /media/btrfs/, fsid bb600f14-9fbb-4f27-af33-95c6ac1975fe (pid=16546)
 sudo btrfs scrub status /media/btrfs/
   scrub status for bb600f14-9fbb-4f27-af33-95c6ac1975fe
	scrub started at Mon Nov 18 13:54:43 2019, running for 00:15:05
	total bytes scrubbed: 84.04GiB with 5340488 errors
	error details: read=5340485 super=3
	corrected errors: 0, uncorrectable errors: 5340485, unverified errors: 0

Efectivament, es troben molts errors que no pot solucionar.

Si fem un smartctl al disc que sembla fallar:

sudo smartctl /dev/sdf -a
smartctl 6.6 2016-05-31 r4324 [x86_64-linux-4.15.0-70-generic] (local build)
Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Seagate Barracuda 7200.14 (AF)
Device Model:     ST2000DM001-1ER164
Serial Number:    W4Z19BCP
LU WWN Device Id: 5 000c50 07d44b288
Firmware Version: CC25
User Capacity:    2.000.398.934.016 bytes [2,00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    7200 rpm
Form Factor:      3.5 inches
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ACS-2, ACS-3 T13/2161-D revision 3b
SATA Version is:  SATA 3.1, 6.0 Gb/s (current: 3.0 Gb/s)
Local Time is:    Mon Nov 18 15:53:14 2019 UTC
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x82)	Offline data collection activity
					was completed without error.
					Auto Offline Data Collection: Enabled.
Self-test execution status:      (   0)	The previous self-test routine completed
					without error or no self-test has ever 
					been run.
Total time to complete Offline 
data collection: 		(   89) seconds.
Offline data collection
capabilities: 			 (0x7b) SMART execute Offline immediate.
					Auto Offline data collection on/off support.
					Suspend Offline collection upon new
					command.
					Offline surface scan supported.
					Self-test supported.
					Conveyance Self-test supported.
					Selective Self-test supported.
SMART capabilities:            (0x0003)	Saves SMART data before entering
					power-saving mode.
					Supports SMART auto save timer.
Error logging capability:        (0x01)	Error logging supported.
					General Purpose Logging supported.
Short self-test routine 
recommended polling time: 	 (   1) minutes.
Extended self-test routine
recommended polling time: 	 ( 219) minutes.
Conveyance self-test routine
recommended polling time: 	 (   2) minutes.
SCT capabilities: 	       (0x1085)	SCT Status supported.

SMART Attributes Data Structure revision number: 10
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000f   117   099   006    Pre-fail  Always       -       162249504
  3 Spin_Up_Time            0x0003   100   096   000    Pre-fail  Always       -       0
  4 Start_Stop_Count        0x0032   100   100   020    Old_age   Always       -       175
  5 Reallocated_Sector_Ct   0x0033   100   100   010    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x000f   079   060   030    Pre-fail  Always       -       89979761
  9 Power_On_Hours          0x0032   081   081   000    Old_age   Always       -       16693
 10 Spin_Retry_Count        0x0013   100   100   097    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   020    Old_age   Always       -       85
183 Runtime_Bad_Block       0x0032   100   100   000    Old_age   Always       -       0
184 End-to-End_Error        0x0032   100   100   099    Old_age   Always       -       0
187 Reported_Uncorrect      0x0032   100   100   000    Old_age   Always       -       0
188 Command_Timeout         0x0032   100   089   000    Old_age   Always       -       1 2 12
189 High_Fly_Writes         0x003a   099   099   000    Old_age   Always       -       1
190 Airflow_Temperature_Cel 0x0022   064   048   045    Old_age   Always       -       36 (Min/Max 30/38)
191 G-Sense_Error_Rate      0x0032   100   100   000    Old_age   Always       -       0
192 Power-Off_Retract_Count 0x0032   100   100   000    Old_age   Always       -       44
193 Load_Cycle_Count        0x0032   067   067   000    Old_age   Always       -       67287
194 Temperature_Celsius     0x0022   036   052   000    Old_age   Always       -       36 (0 9 0 0 0)
197 Current_Pending_Sector  0x0012   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0010   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x003e   200   069   000    Old_age   Always       -       262
240 Head_Flying_Hours       0x0000   100   253   000    Old_age   Offline      -       16485h+48m+15.949s
241 Total_LBAs_Written      0x0000   100   253   000    Old_age   Offline      -       19632474908
242 Total_LBAs_Read         0x0000   100   253   000    Old_age   Offline      -       1261379460714

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
No self-tests have been logged.  [To run self-tests, use: smartctl -t]

SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.

Diu que no està molt sà, però que funciona. Per tant, cal intentar reparar el sistema d'arxius:

btrfs rescue zero-log /dev/sdf1  # Si és un error de transaccions tallades per un tall elèctric, per exemple.
btrfs rescue super-recover       # Si no està el superblock, el recupera de una de les còpies que té el disc.
btrfs rescue chunk-recover       # En cas de fallar les metadates i tindre fitxers desconeguts. És lent i perillós.
btrfs check --repair /dev/sdXY   #  Per reparar el sistema d'arxius.