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1. 개요

• LVM Cache (lvconvert cache) 기능을 활용하여 저속 디스크(vdb1) 위에 생성한 LV를 고속 디스크(vdc1)기반 Cache Pool로 가속하는 과정을 다룹니다.

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2. 버전

• Rocky Linux 9.5

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3. 설명

3-1. LVM Cache란?

• LVM Cache는 **저속 디스크(HDD)**에 위치한 Logical Volume(LV)을 **고속 디스크(SSD/NVMe)**를 활용해 캐싱(caching) 하는 기능입니다.

3-2. LVM Cache 동작 방식

• LVM Cache는 기본적으로 **원본 LV(origin)**와 **캐시 풀(cache pool)**을 결합해 동작합니다.
  1. 구성 요소
     • Origin LV: 실제 데이터가 저장되는 저속 디스크(HDD)의 LV
     • Cache Pool: 캐시 역할을 하는 고속 디스크(SSD/NVMe)
       • Cache Data (실제 캐싱된 데이터 블록 저장)
       • Cache Metadata (어떤 블록이 캐싱되었는지 추적)
  2. Cache 모드
     • Writeback 모드 (권장)
       • 쓰기 요청이 SSD에 먼저 기록 → 이후 비동기적으로 HDD에 반영(sync).
       • 장점: 쓰기 성능이 크게 향상됨.
       • 단점: SSD 장애 시 최근 데이터 손실 가능성 존재.
     • Writethrough 모드
       • 모든 쓰기를 SSD와 HDD에 동시에 기록.
       • 장점: 데이터 안정성 ↑
       • 단점: 쓰기 성능은 HDD 수준에 머무름.
  3. Cache 정책 (Policy)
     • smq (Stochastic Multiqueue) : 기본값, 자주 쓰이는 데이터를 효율적으로 캐싱.
     • mq : 구 버전 정책, 현재는 잘 쓰이지 않음.
  4. 동작 흐름
     • 읽기(Read):
       • SSD(Cache)에 있으면 → 빠른 응답 (Cache Hit).
       • SSD에 없으면 → HDD에서 가져오고 SSD에 적재 (Cache Miss + Promotion).
     • 쓰기(Write):
       • Writeback 모드: SSD → 나중에 HDD 반영.
       • Writethrough 모드: SSD와 HDD 동시 기록.

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4. Partition

4-1. Partition 생성

# parted -s /dev/vdb mklabel gpt
# parted -s /dev/vdb mkpart vdb 1 100%
# parted -s /dev/vdb set 1 lvm on

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5. PV(Physical Volume)

5-1. PV(Physical Volume) 생성

# pvcreate /dev/vdb1

5-2. PV(Physical Volume) 생성 확인

# pvdisplay -s

출력 예시:

  Device "/dev/vdb1" has a capacity of <128.00 GiB

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6. VG(Volume Group)

6-1. VG(Volume Group) 생성

# vgcreate vg_test /dev/vdb1

6-2. VG(Volume Group) 생성 확인

# vgdisplay

출력 예시:

  --- Volume group ---
  VG Name               vg_test
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  1
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                0
  Open LV               0
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               <128.00 GiB
  PE Size               4.00 MiB
  Total PE              32767
  Alloc PE / Size       0 / 0
  Free  PE / Size       32767 / <128.00 GiB
  VG UUID               gmjgrB-CGSb-iUmT-qd3I-QThR-Au2V-fihkA1

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7. LV(Logical Volume)

7-1. LV(Logical Volume) 생성

# lvcreate -n lv_ext4 -l 32767 vg_test

7-2. LV(Logical Volume) 생성 확인

# lvdisplay

출력 예시:

  --- Logical volume ---
  LV Path                /dev/vg_test/lv_ext4
  LV Name                lv_ext4
  VG Name                vg_test
  LV UUID                fYd8cT-3u5x-t26c-19Sf-9yxp-llo3-CT54Y9
  LV Write Access        read/write
  LV Creation host, time KVM01, 2025-09-06 17:08:09 +0900
  LV Status              available
  # open                 0
  LV Size                <128.00 GiB
  Current LE             32767
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           252:0

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8. 파일 시스템

8-1. 파일 시스템 생성

# mkfs.ext4 /dev/vg_test/lv_ext4

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9. 마운트

9-1. 마운트 포인트 생성

# mkdir /mnt/ext4

9-2. 마운트

# mount /dev/vg_test/lv_ext4 /mnt/ext4/

9-3. 마운트 확인

# df -Th /mnt/ext4/

출력 예시:

Filesystem                  Type  Size  Used Avail Use% Mounted on
/dev/mapper/vg_test-lv_ext4 ext4  126G   24K  120G   1% /mnt/ext4

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10. Partition

10-1. Partition 생성

# parted -s /dev/vdc mklabel gpt
# parted -s /dev/vdc mkpart vdc 1 100%
# parted -s /dev/vdc set 1 lvm on

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11. PV(Physical Volume)

11-1. PV(Physical Volume) 생성

# pvcreate /dev/vdc1

출력 예시:

  Physical volume "/dev/vdc1" successfully created.

11-2. PV(Physical Volume) 생성 확인

# pvdisplay -s

출력 예시:

  Device "/dev/vdb1" has a capacity of 0
  Device "/dev/vdc1" has a capacity of <128.00 GiB

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12. VG(Volume Group)

12-1. VG(Volume Group) 확인

# vgdisplay -s

출력 예시:

  "vg_test" <128.00 GiB [<128.00 GiB used / 0    free]

12-2. VG(Volume Group) 확장

# vgextend vg_test /dev/vdc1

출력 예시:

  Volume group "vg_test" successfully extended

12-3. VG(Volume Group) 확장 확인

# vgdisplay -s

출력 예시:

  "vg_test" 255.99 GiB [<128.00 GiB used / <128.00 GiB free]

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13. LV(Logical Volume)

13-1. Cache Pool 생성

# lvcreate --type cache-pool -n cachepool -L 100G vg_test /dev/vdc1

출력 예시:

  Using 128.00 KiB chunk size instead of default 64.00 KiB, so cache pool has less than 1000000 chunks.
  Logical volume "cachepool" created.

13-2. Cache Pool 생성 확인

# lvdisplay vg_test/cachepool

출력 예시:

  --- Logical volume ---
  LV Path                /dev/vg_test/cachepool
  LV Name                cachepool
  VG Name                vg_test
  LV UUID                XU13p3-mOHa-vM0Q-IuQ3-Baon-m9hC-NksTly
  LV Write Access        read/write
  LV Creation host, time KVM01, 2025-09-06 17:14:06 +0900
  LV Pool metadata       cachepool_cmeta
  LV Pool data           cachepool_cdata
  LV Status              NOT available
  LV Size                100.00 GiB
  Current LE             25600
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto

13-3. LV Cache

13-3-1. LV Cache 적용 전 확인

# lvdisplay -m /dev/vg_test/lv_ext4

출력 예시:

  --- Logical volume ---
  LV Path                /dev/vg_test/lv_ext4
  LV Name                lv_ext4
  VG Name                vg_test
  LV UUID                UlcPen-hyeP-mDfM-pnd1-jGLE-RtdW-Dp2XnI
  LV Write Access        read/write
  LV Creation host, time KVM01, 2025-09-07 12:00:36 +0900
  LV Status              available
  # open                 1
  LV Size                <128.00 GiB
  Current LE             32767
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           252:0

  --- Segments ---
  Logical extents 0 to 32766:
    Type                linear
    Physical volume     /dev/vdb1
    Physical extents    0 to 32766

# lvs -a -o lv_name,segtype,cache_mode,cache_policy,metadata_percent,data_percent,devices vg_test

출력 예시:

  LV                Type       CacheMode CachePolicy Meta%  Data%  Devices
  cachepool         cache-pool                                     cachepool_cdata(0)
  [cachepool_cdata] linear                                         /dev/vdc1(20)
  [cachepool_cmeta] linear                                         /dev/vdc1(10)
  lv_ext4           linear                                         /dev/vdb1(0)
  [lvol0_pmspare]   linear                                         /dev/vdc1(0)

13-3-2. LV Cache 적용

# lvconvert --type cache --cachepool vg_test/cachepool --cachemode writeback vg_test/lv_ext4

출력 예시:

Do you want wipe existing metadata of cache pool vg_test/cachepool? [y/n]: **y**
  Logical volume vg_test/lv_ext4 is now cached.

13-3-3. LV Cache 적용 확인

# lvdisplay -m /dev/vg_test/lv_ext4

출력 예시:

  --- Logical volume ---
  LV Path                /dev/vg_test/lv_ext4
  LV Name                lv_ext4
  VG Name                vg_test
  LV UUID                UlcPen-hyeP-mDfM-pnd1-jGLE-RtdW-Dp2XnI
  LV Write Access        read/write
  LV Creation host, time KVM01, 2025-09-07 12:00:36 +0900
  LV Cache pool name     cachepool_cpool
  LV Cache origin name   lv_ext4_corig
  LV Status              available
  # open                 1
  LV Size                <128.00 GiB
  Cache used blocks      0.01%
  Cache metadata blocks  16.14%
  Cache dirty blocks     0.00%
  Cache read hits/misses 5 / 34
  Cache wrt hits/misses  0 / 0
  Cache demotions        0
  Cache promotions       4
  Current LE             32767
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     512
  Block device           252:0

  --- Segments ---
  Logical extents 0 to 32766:
    Type                cache
    Chunk size          128.00 KiB
    Metadata format     2
    Mode                writeback
    Policy              smq

# lvs -a -o lv_name,segtype,cache_mode,cache_policy,metadata_percent,data_percent,devices vg_test

출력 예시:

  LV                      Type       CacheMode CachePolicy Meta%  Data%  Devices
  [cachepool_cpool]       cache-pool writeback smq         16.14  0.01   cachepool_cpool_cdata(0)
  [cachepool_cpool_cdata] linear                                         /dev/vdc1(20)
  [cachepool_cpool_cmeta] linear                                         /dev/vdc1(10)
  lv_ext4                 cache      writeback smq         16.14  0.01   lv_ext4_corig(0)
  [lv_ext4_corig]         linear                                         /dev/vdb1(0)
  [lvol0_pmspare]         linear                                         /dev/vdc1(0)

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14. 확인

14-1. 패키지 설치

# dnf -y install sysstat

14-2. iostat 실행

# iostat -x /dev/vdb1 /dev/vdc1 1

14-3. 쓰기 테스트

# dd if=/dev/zero of=/mnt/ext4/test  file bs=1024M count=100 oflag=direct

14-4. 확인

• 쓰기 시 캐시 디스크(vdc1)의 w/s 값이 올라가는 것을 확인 하실 수 있습니다.

• 쓰기가 완료 된 다음 캐시 디스크는 r/s 값이 올라가는 것을 확인 하실 수 있고, 원본 LV 디스크는 w/s 값이 올라가는 것을 확인 하실 수 있습니다.

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