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Need help with very slow drive

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I am running unraid 5.0.5, and recently I've noticed that my read performance is terrible (at times).  I decided to run some diagnostics using hdparm -tT. 


Here is the result for one of my data drives:



Timing cached reads:  1038 MB in  2.00 seconds = 519.20 MB/sec

Timing buffered disk reads:  12 MB in  3.16 seconds =  3.79 MB/sec


This is way out of line with the other drives, such as:



Timing cached reads:  1490 MB in  2.00 seconds = 744.54 MB/sec

Timing buffered disk reads: 418 MB in  3.01 seconds = 138.67 MB/sec


Here is the hdparm info for the slow drive:



ATA device, with non-removable media
        Model Number:       ST31000528AS
        Serial Number:      6VP240NB
        Firmware Revision:  CC38
        Transport:          Serial
        Used: unknown (minor revision code 0x0029)
        Supported: 8 7 6 5
        Likely used: 8
        Logical         max     current
        cylinders       16383   16383
        heads           16      16
        sectors/track   63      63
        CHS current addressable sectors:   16514064
        LBA    user addressable sectors:  268435455
        LBA48  user addressable sectors: 1953525168
        Logical/Physical Sector size:           512 bytes
        device size with M = 1024*1024:      953869 MBytes
        device size with M = 1000*1000:     1000204 MBytes (1000 GB)
        cache/buffer size  = unknown
        Nominal Media Rotation Rate: 7200
        LBA, IORDY(can be disabled)
        Queue depth: 32
        Standby timer values: spec'd by Standard, no device specific minimum
        R/W multiple sector transfer: Max = 16  Current = ?
        Recommended acoustic management value: 254, current value: 0
        DMA: mdma0 mdma1 mdma2 udma0 udma1 udma2 udma3 udma4 udma5 *udma6
             Cycle time: min=120ns recommended=120ns
        PIO: pio0 pio1 pio2 pio3 pio4
             Cycle time: no flow control=120ns  IORDY flow control=120ns
        Enabled Supported:
           *    SMART feature set
                Security Mode feature set
           *    Power Management feature set
           *    Write cache
           *    Look-ahead
           *    Host Protected Area feature set
           *    WRITE_BUFFER command
           *    READ_BUFFER command
           *    DOWNLOAD_MICROCODE
                SET_MAX security extension
           *    Automatic Acoustic Management feature set
           *    48-bit Address feature set
           *    Device Configuration Overlay feature set
           *    Mandatory FLUSH_CACHE
           *    FLUSH_CACHE_EXT
           *    SMART error logging
           *    SMART self-test
           *    General Purpose Logging feature set
           *    WRITE_{DMA|MULTIPLE}_FUA_EXT
           *    64-bit World wide name
                Write-Read-Verify feature set
           *    WRITE_UNCORRECTABLE_EXT command
           *    {READ,WRITE}_DMA_EXT_GPL commands
           *    Segmented DOWNLOAD_MICROCODE
           *    Gen1 signaling speed (1.5Gb/s)
           *    Gen2 signaling speed (3.0Gb/s)
           *    Native Command Queueing (NCQ)
           *    Phy event counters
                Device-initiated interface power management
           *    Software settings preservation
           *    SMART Command Transport (SCT) feature set
           *    SCT Read/Write Long (AC1), obsolete
           *    SCT Write Same (AC2)
           *    SCT Error Recovery Control (AC3)
           *    SCT Features Control (AC4)
           *    SCT Data Tables (AC5)
                unknown 206[12] (vendor specific)
        Master password revision code = 65534
        not     enabled
        not     locked
        not     frozen
        not     expired: security count
                supported: enhanced erase
Logical Unit WWN Device Identifier: 5000c5001b892a49
        NAA             : 5
        IEEE OUI        : 000c50
        Unique ID       : 01b892a49
Checksum: correct


I see no obvious explanation for this.  A recent parity check came back clean.  Any ideas?  Could it be a bad SATA cable?  Something like that?

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OK, I decided to shut the array down and swap out the SATA cable.  This made no difference in hdparm read performance.  Then, I swapped out with a spare drive and tested this new drive with hdparm and got much better results.  These tests indicate a problem with the original hard drive, so I swapped it out of the array and I am not rebuilding from parity. 

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