Низкая скорость в raid-0

$ cat /proc/mdstat
md1 : active raid0 sda3[0] sdb3[1] sdc3[2] sdd3[3]
      33768192 blocks 64k chunks

$ hdparm -I /dev/sda

/dev/sda:

ATA device, with non-removable media
        Model Number:       ST31500341AS                            
        Serial Number:      9VS3T5V2
        Firmware Revision:  CC1H    
        Transport:          Serial
Standards:
        Used: unknown (minor revision code 0x0029) 
        Supported: 8 7 6 5 
        Likely used: 8
Configuration:
        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: 2930277168
        Logical/Physical Sector size:           512 bytes
        device size with M = 1024*1024:     1430799 MBytes
        device size with M = 1000*1000:     1500301 MBytes (1500 GB)
        cache/buffer size  = unknown
        Nominal Media Rotation Rate: 7200
Capabilities:
        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 = 16
        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
Commands/features:
        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 Long Sector Access (AC1)
           *    SCT LBA Segment Access (AC2)
           *    SCT Error Recovery Control (AC3)
           *    SCT Features Control (AC4)
           *    SCT Data Tables (AC5)
                unknown 206[12] (vendor specific)
Security: 
        Master password revision code = 65534
                supported
        not     enabled
        not     locked
                frozen
        not     expired: security count
                supported: enhanced erase
        256min for SECURITY ERASE UNIT. 256min for ENHANCED SECURITY ERASE UNIT.
Logical Unit WWN Device Identifier: 5000c50020027b9c
        NAA             : 5
        IEEE OUI        : 000c50
        Unique ID       : 020027b9c
Checksum: correct

$ dd if=/dev/zero of=ddfile bs=1M count=500
500+0 записей считано
500+0 записей написано
 скопировано 524288000 байт (524 MB), 1.20757 c, 434 MB/c
$ dd if=ddfile of=/dev/null bs=1M count=500
500+0 записей считано
500+0 записей написано
 скопировано 524288000 байт (524 MB), 0.0965052 c, 5.4 GB/c
$ dd if=/dev/zero of=ddfile bs=1M count=5000
5000+0 записей считано
5000+0 записей написано
 скопировано 5242880000 байт (5.2 GB), 22.5937 c, 232 MB/c
$ dd if=ddfile of=/dev/null bs=1M count=5000
5000+0 записей считано
5000+0 записей написано
 скопировано 5242880000 байт (5.2 GB), 24.1826 c, 217 MB/c

echo 3 > /proc/sys/vm/drop_caches

tune2fs -l /dev/sdz | grep "Block size:"

$ echo 3 > /proc/sys/vm/drop_caches
$ sync
$ dd if=/dev/zero of=ddfile bs=1M count=5000
5000+0 записей считано
5000+0 записей написано
 скопировано 5242880000 байт (5.2 GB), 23.5058 c, 223 MB/c
$ echo 3 > /proc/sys/vm/drop_caches
$ sync
$ dd if=ddfile of=/dev/null bs=1M count=5000
5000+0 записей считано
5000+0 записей написано
 скопировано 5242880000 байт (5.2 GB), 23.675 c, 221 MB/c

$ dd if=/dev/zero of=ddfile bs=4M count=1000
1000+0 записей считано
1000+0 записей написано
 скопировано 4194304000 байт (4.2 GB), 18.25 c, 230 MB/c
$ dd if=ddfile of=/dev/null bs=4M count=1000
1000+0 записей считано
1000+0 записей написано
 скопировано 4194304000 байт (4.2 GB), 0.738789 c, 5.7 GB/c
$ dd if=/dev/zero of=ddfile bs=6M count=1000
1000+0 записей считано
1000+0 записей написано
 скопировано 6291456000 байт (6.3 GB), 27.8271 c, 226 MB/c
$ dd if=ddfile of=/dev/null bs=6M count=1000
1000+0 записей считано
1000+0 записей написано
 скопировано 6291456000 байт (6.3 GB), 30.6888 c, 205 MB/c

$ dd if=ddfile of=/dev/null bs=6M count=1000 iflag=direct
1000+0 записей считано
1000+0 записей написано
 скопировано 6291456000 байт (6.3 GB), 31.0574 c, 203 MB/c

$ dd if=/dev/zero of=ddfile bs=6M count=1000 oflag=direct
1000+0 записей считано
1000+0 записей написано
 скопировано 6291456000 байт (6.3 GB), 30.8944 c, 204 MB/c

$ dd if=ddfile of=/dev/null bs=6M count=1000 iflag=direct
1000+0 записей считано
1000+0 записей написано
 скопировано 6291456000 байт (6.3 GB), 29.306 c, 215 MB/c